He met Rachel at her college graduation ceremony in 2011, ________ he was later to marry.
A．which B．when
C．where D．whom
—When did you get home last night?
—It was almost midnight ________ we arrived home.
A．that B．before
C．since D．when
________ is known to all is that effective measures should be taken to deal with the problem of the overexploitation of our resources.
A．What B．It
C．As D．Which
I think you should work harder ________ you are satisfied with the way things are.
A．if B．when
C．since D．unless
．Ram explained to his mother ________ he would get home late that night because of extra work.
A．that B．which
C．what D．whether
Air pollution is getting more and more serious, so we must take action ________ it is too late.
A．before B．after
C．until D．when
