20. _______, English is one of the most important subjects.
| A．It is known to all that | B．As is known to all that |
| C．As we know | D．As it is known to all |
I remember a ______ famous scientist said that we should believe in ourselves first of all.
| A．some | B．certain | C．sure | D．other |
We must not touch this switch under any ______.
| A．situation | B．atmosphere |
| C．circumstances | D．civilization |
I had written twice to Janet. But she didn't ______ my letters, so I decided to go there myself.
| A．return | B．reply | C．answer | D．receive |
You've ______ a zero in this phone number, so you can't get through.
| A．left out | B．broken out |
| C．put out | D．got out |
He did not regret saying what he did but felt that he ______ it differently.
| A．could express | B．would express |
| C．could have expressed | D．must have expressed |