The mad man was put in the soft padded cell last he __ himse

The mad man was put in the soft padded cell last he __ himself.
A should injure B injured C had injured D would injure
答案是A但是D也不好排除,为什么D不能选

这个句子里的last 应该是lest
lest的意思是唯恐,以免,免得.它是连词.
它所连接的状语从句里常用should或原形动词,意思：以免; 生怕; 唯恐 ,这是虚拟语气
例如：
Be careful lest you fall from that tree.
要当心,以免从树上摔下来.
I obeyed her lest she should be angry.
我得顺着她,免得她生气.
跟这个词同样用法的还有：in case（以防）和 for fear that（生怕,唯恐）