1.how do you know if milk has -----bad without tasting it or

1.how do you know if milk has _____bad without tasting it or smelling it
A.tasted
B.smelled
C.gone
D.stayed
(我知道AB两个选项不对,那C,D应该选哪一个呢?
2.Sometimes in spring ,the old leaves from last year ______yellow and drop off.
A.remain
B.keep
C.looks
D.turn
(这四个选项我都不大清楚,应该选哪一个呢?）
更正一下第二个题目的第三个选项打错了应该是seem。