______ he wasn't ready in time, we had to carry out the plan without him.
 
【正确答案】 B
【答案解析】 根据句意可知,空处引导原因状语从句,故用连词as,表示“既然,由于”。because of表示“因为,由于”时,后跟名词、代词或相当于名词的短语,不能接句子。