It was until last year that he ________.
A.left his home town for a new start
B.came to realize the importance of learning English
C.worked as an English teacher at a middle school
D.set out to build a new house of his own
请详解!

选择C

首先这是一个强调句句型， 横线部分应直接加陈述句（一般过去时）。
其次，你可以把它还原成一般句来选择，he ______ until last year 。
再次，until +时间点 用在否定句中一般表示肯定的意义，如：
My father didn't come back until 10 o'clock.
我爸爸10点才回来。
而
I did my homework until 10 o'clock.
我做作业一直做到10点。
所以until +时间点 用在肯定句中一般表示一种动作的持续，所以，上题应选择C，意思是
他在一所中学一直当英语老师，一直当到去年。
其他三个选项都是瞬间性动作，所以不选。