(1)When do we need to pay the balance?

(1)When do we need to pay the balance?
____September 30
A.In B.By C.During D.Within
(2)Where did you put the car keys?
Oh,I___put them on the chair because the phone rang as I___in.
A.remember,come B.remembered,was coming
C.remember,come D.remember,was coming
答案分别是B和C.我不理解,望高手说下原因
1 我选的是A···
your balance cast little money，please recharge IN time
那为什么不选IN呢？