When he saw the train moving ,he realized he could not get on it.( D)

题目

When he saw the train moving ,he realized he could not get on it.( D)
A.heard B.achieved C.find D.understood
为什么要选 D ,C为什么不可以,
真不好意思:就打括号换单词
When he saw the train moving ,he (realized )he could not get on it.( D)
A.heard B.achieved C.find D.understood
括号里为什么要换成D,换成C不可以吗?

答案

空在哪哪?
我不得不笑,c时态错了!