The most important thing _____we should pay attention to is

The most important thing _____we should pay attention to is the first thing____I have said.
A.which ,that
B.that,which
C.which ,which
D.that ,that
2.She spent the whole evening talking about the things and persons_____none of us has ever heard of.
A.which B who C.whom D.that
3.__was expected ,he failed in the exam.
A.That B.As C.Which D.It
4.The children climbed up the hill,_____they picnicked.
A.on its top B.on the top of it C.on whose top D.on the top of that