the clock slowed down.it ___ 1 was slow 2 was behind 3 went back 4 went slowly

题目

the clock slowed down.it ___ 1 was slow 2 was behind 3 went back 4 went slowly
请问选择哪一个,及分别说出每个为什么对与错,

答案

It was slow.它走慢了.／ 它是慢.
It went slowly.它慢慢地走.