【题目】We all know that _____ carefully dealt with, the situation will get worse.
A.once
B.when
C.if
D.unless

【答案】D
【解析】考查省略句。主从句主语一致，从句中谓语动词为系动词be，可省略主语和系动词be，完整的句子为We all know that, if the situation carefully dealt with，the situation will get worse．故选D.