The headmaster went to Washington ______ the first time last September.

单项选择题I'm tired out. I ______ all afternoon and I don't seem to have finished anything.

单项选择题一般而言，中学英语单元教材可以分为三大系统，它们主次分明，______是源，作业系统和图表系统是流，三者相辅相成。

单项选择题A bank is the place ______ they lend you an umbrella in fair weather and ask for it back when it begins to rain.

单项选择题In spoken English of some areas in the US, the r sounds at the end of the words（）

A. are dropped
B. drop
C. are being dropped
D. have dropped

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