4. the rest was → the rest were

3. the United States → that of (in) the United States

2. to explain → explain

1. reply →　 answer, 或将reply改为reply to

5. Without knowledge of science and technology, it is impossible to build ours (our country)into a powerful (strong) socialist country.

4. I asked him which of (all)the cities he had heen to (had visited)had the largest population.

3. 1)It takes more than (over)twenty hours to get (go) to Paris by plane (byair).

2) It takes more than (over) twenty hours to fly to Paris.

2. How did you celebrate May Day(May lst) this year?

1. Beijing (Peking)is the capital of China (China' s capital).