study hard,_______ you will get better grade.

study hard,_______ you will get better grade.
A.or B.and C.because D.butt
he rest of the students ______ not allowed to leave the classroom at that time.
A.are B.have C.was D.were
he used to be afraid of snakes.how about you?——so _____i.
A.did B.have C.was D.do
there are _______ trees on that mountain.
A.two millions B.two million of C.million of D.millions of