The letters for the boss on his desk but he didn’t read them until three days later.
A. was put
B. put
C. has put
D. were put
网友回答
A. was put
B. put
C. has put
(答案→)D. were put
解析：考查被动语态。本句的主语是the letters，与动词put构成被动关系，故使用过去分词的形式。句意：给老板的信被放在桌子上，但是知道3天以后他才看见。故D正确。