Theme selection:
First of all, you should be clear about the requirements of the school you want to apply for. Not all American universities require SAT2 scores, and quite a few ordinary universities and a few famous schools do not require SAT2. If the school you are applying for doesn't need SAT2 scores, and you are pressed for TOEFL and SAT 1, then there is no need to take SAT2. However, most famous American schools need to have SAT2 scores, and generally need two or three courses. In schools that need SAT2 scores, the requirements for specific subjects are different. Some schools do not specify subjects, while others give a considerable degree of designation. For example, Caltech requires all applicants to provide SAT2, Math 2 and a science category (i.e. physics or chemistry or biology).
In addition, in addition to the general enrollment regulations of schools, different departments generally have their own requirements for SAT2 exam subjects, which is often referred to as "college-specific" in the enrollment policies of various schools. Therefore, when choosing the SAT2 exam subjects, you must be clear about the requirements of the school you apply for and the department you apply for. Generally speaking, the requirements of a school or department for SAT2 subjects are compatible with the professional style of the school or department itself. For example, many famous universities of science and technology require compulsory mathematics, and the departments of materials and electronics often require compulsory physics or chemistry. Of all the 20 SAT2 exams, it is very unlikely for China candidates to take the German, French and other language exams (Chinese can't take it because it is their mother tongue), and English, that is, English literature exam, is not recommended because it is too difficult. Of the two history exams, the world history exam is not recommended because it involves too many test sites. American history examination can be regarded as a choice for liberal arts students with poor science foundation or students who are going to study social sciences in the United States.
For most candidates in China, especially science students, it is most likely to choose two exams in mathematics and three exams in science. Their examination scope is basically the same as that of domestic high school graduates, and some knowledge points are beyond, so it is a necessary prerequisite to have knowledge reserves of corresponding subjects in high school. There are two exams to choose from in mathematics: Math One and Math Two. Math II is a little more difficult than math, but it is much lower than the national college entrance examination and even the national examination in China. Domestic candidates can generally get full marks. Therefore, if you take the math test, you should choose Math II. After all, some schools, such as California Institute of Technology mentioned above, explicitly require Math II.
In the three exams of science, it is recommended that you take physics or chemistry.
First of all, there are many majors related to physics and chemistry, which have a wide range of applications. For example, most engineering majors such as physics and electronic engineering, computer, machinery and electrical engineering are related, while chemistry is related to materials, polymers, biology and pharmacy, while biology examination only corresponds to biology and medicine. When applying for these two departments, you can often get results in biology or chemistry.
Secondly, in terms of professional vocabulary, some insects, tissues and organs, plant names, etc. Everything that appears in the biology exam is complicated, and it is very troublesome to prepare. However, physics or chemistry involves much less professional vocabulary, especially physics. The average difficulty of physical and chemical examination is much lower than that of similar examinations in China, but there are some test sites involved that China students have not studied, such as red shift in physics, big bang theory, standing wave formula, uncertainty principle, extranuclear electron configuration's law in chemistry, preliminary thermodynamics and so on. Some of these knowledge points are the course contents of China University, and some belong to popular science. They are all SAT2 exams, but the topics are relatively easy to write, which reflects the characteristics that American basic education pursues breadth rather than difficulty.
In addition to the necessary knowledge reserve and professional vocabulary, reading ability is also the key to getting a high score on SAT2, which is particularly important for China students. SAT2 takes one hour for all subjects, which is enough for China students' best mathematics, but it is not easy for physics, chemistry, biology, history and other exams. SAT2 has 75 physical questions, about 70 chemical questions, 80 biological questions (one of the biochemistry or ecology volumes) and 90 American history questions. And there are many subjects with relatively long explanatory questions. For candidates with poor English foundation and poor reading ability, it is not easy to read more than 70 questions correctly and without confusion in one hour, let alone think and answer. Therefore, it is very important to improve reading ability, including basic vocabulary, reading speed, ability to read long sentences and ability to master key words.