More and more people have a low-C life and the air is getting much

    than a few years ago.

A．clean

B．cleanest

C．cleaner

D．the cleanest

受体拮抗剂的特点是（）

A.与受体有亲和力而无内在活性

B.对受体无亲和力而有内在活性

C.对受体有亲和力和内在活性

D.对受体的亲和力大而内在活性小

E.对受体的内在活性大而亲和力小