Flight attendants usually need to arrive at the airport ______ before their flight leaves.
I have bought a good book, ______ that will surely interest of the students.
The student went through the paper, finding some mistakes that might have been ignored.
When Tracy was told her son's illness could be cured, she left the hospital .
He is quite strange, for everything he does is opposite to ______ is considered normal behavior.
Ancient Greeks were responsible for many of the _______ in science and other fields.