21．The moment we heard the news of the earthquake, we began to ________ the work to rescue those who were trapped in the building.
| A．set off | B．set down | C．set aside | D．set about |
________ he is well again, he can go on with his English study.
| A．Ever since | B．Now that | C．So that | D．As if |
This is not an economical way to get more water; _________,it is very expensive.
| A．on the contrary | B．on the other hand | C．in a word | D．rather than |
After her son’s death, the old lady __________ sit at the window, deep in thought.
| A．would | B．should | C．could | D．might |
The beautiful stone _________ as much as 2.5 tons was placed in Mr. White’s garden last week.
| A．weighed | B．weighing | C．to be weighed | D．being weighed |
--- I didn’t believe you made such a mistake.
---Oh, I’m sorry. I did it ___________. I’m serious.
| A．on purpose | B．by accident | C．by the way | D．as for |