问题补充：
— What was it that caused the party to be put off?
— the invitations.
A. Because Tom delayed sending
B. Tom delayed to send
C. That Tom delayed sending
D. Tom delaying sending
答案：
A. Because Tom delayed sending
B. Tom delayed to send
(答案→)C. That Tom delayed sending
D. Tom delaying sending
解析：考查句子的省略形式，根据句子结构可知，该句为主语从句的省略形式。完整为That Tom delayed sending the invitations caused the party to be put off.
