受体调节的变化可以是()。
A.受体数量的增加或减少
B.同系调节
C.异系调节
D.配基亲和力的升高或降低
E.以上均不正确
参考答案:A, D
She is a diligent student, and all her spare time she has _____ staying in the library.
A.is spent
B.spent
C.to spend
D.to be spent
冬天,在贮存蔬菜的地窖里放上几桶水,这是利用______________来防止蔬菜被冻坏。