By the time I was nine ,I ______hundreds of English words.

A．had learned

B．have learned

C．has been learned

D．had been learned

实验证明，相同形状的不同材质的导体其电阻的数值不同，（）的电阻率较小。

A．金属

B．木材

C．玻璃

D．陶瓷