The teacher told us to remain silent unless _______.

Some people are worrying about the possibility ________ mobile phones could be responsible for a range of illnesses, from rashes(皮疹) to brain tumors

I wonder if your father _______ you to go swimming.— I’m not sure. But I will tell you if he _______ to let me join you.

ternetcelebrated its birthday on September 2nd, by which time 40 years _______ sincethe first successful transfer of data between two computers

Martin drove along ______ he found a convenientparking place．