＿＿＿used to be thought that the earth was square. 

The field research will take Joan and Paul about five months; it will be a long time _________ we meet them again.

 They have done much of the work; when will ________ be finished?

I think Mrs. Stark could be _______ between 50 and 60 years of age.

 –How many students are there in the classroom? --__________. They are all in the lab.