If it is difficult, choose "cuddling" and "class homework"! "cuddling" specializes in training some error-prone questions, some of which are quite difficult! If you want something more difficult, choose class assignments! Remember that it is a classic study, don't buy a revised edition, the topic will make you cool, and if you fully understand it, your grades will definitely be sluggish! These are for my own use, please leave a message if you have any questions!