A. are droppedB. dropC. are being droppedD. have droppe……

单项选择题For many years, people （） electric cars. However, making them has been more difficult that predicted.

A. had dreamed of
B. have dreamed of
C. dreamed of
D. dream of

单项选择题I have to delay（）the letter.

A. to answer
B. answering
C. answer
D. answered

单项选择题教学技术的使用必须按照______来决定。