—What were you doing when Tony phoned you?

—I had just finished my work and________ to take a shower.

A．had started

B．started

C．have started

D．was starting

头孢菌素类的抗菌机制是（）

A.抑制细菌蛋白质合成

B.抑制细菌细胞壁合成

C.与PABA竞争二氢叶酸合成酶

D.抑制二氢叶酸还原酶

E.抑制DNA回旋酶