We （） on this project for four hours. Let's have a rest.

单项选择题There is a ______ of impatience in the tone of his voice.

单项选择题The air of countryside is cleaner than ______ of city, but I don't like to live ______ far.

单项选择题We（）on this project for four hours. Let's have a rest.

A. are working
B. have been working
C. worked
D. had worked

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This area of the park has been specially ______ for children, but accompanying adults are also welcome.