如果which指代的先行词是物，根据物的性质，可以是单数，也可以是复数。如：
- She gave me a book, which is expensive.
- She gave me some books, which are bought from a nearby bookshop.
但which指代的先行词也可能是前面整个一个句子，那个这个时候which后面的谓语动词只能用单数。如：
- She got an A in the exam, which was due to her diligence.
