I ________ work out the problem at first, but after my teacher showed me an example, I ________ get the answer easily.

A．couldn’t; was able to

B．can’t; could

C．wasn’t able to; could

D．was able to; was able to

虎杖中具有镇咳及降血脂作用的成分为

A．大黄素
B．大黄酚
C．土大黄苷
D．白藜芦醇
E．虎杖苷