高中英语主谓一致单选题

Listening attentively in class and reviewing in time _______ a sure way to improve your learning, and this is especially true _______ it comes to standard tests.

A．is, when

B．are, when

C．is, while

D．are, as

---You could have walked here.
---Yes. A taxi ___________ necessary.

A．is

B．was

C．isn’t

D．wasn’t

The factory used 65 percent of the raw materials, the rest of which _____ saved for the other purpose.

A．is

B．are

C．was

D．were