The fact has worried many scientists ______ the earth is becoming warmer and warmer these years.
| A．what | B．which | C．that | D．though |
The chairman thought _______ necessary to invite Professor Smith to speak at the meeting .
| A．that | B．it | C．this | D．him |
------Flight 302 to London_______ until 7:30 tomorrow morning.
-----No wonder I hear so many complaints from the passengers.
| A．has been delayed | B．delay | C．delayed | D．will be delayed |
When the American constitution ______into effect in 1789,it didn’t contain any words guaranteeing the basic rights of ordinary citizens.
| A．went | B．had gone | C．has gone | D．was going |