1.a is the subject of this sentence, and the five used books after with are used to modify the previous library, so the subject is singular, and the three bills to be used are singular in the third person, so 2. The first paper is notes, experience, the second paper is paper, and the paper is papermaking. Like the first question, the theme is mother, mother and her son, so it is singular, so it is was4. Due to deforestation and destruction, a large number of deserts cover the ground. 5. So that sentence, ... even 6. It's not just a collocation of one ... other (s), here is another meaning. 7. The principle of proximity depends on the nearest object.