Question

Helen _______her keys in the office so she had to wait until

Answer 1

C

考查时态。had to提示是一般过去时，发生在“过去的过去”的事情，用过去完成时，排除A、B；until引导的时间状语从句的时态要与主句一致，故用过去时，排除D。故选C。