____my mother couldn’t come back on time,I had to turn to Mary f

____my mother couldn’t come back on time,I had to turn to Mary for help.
A、until
B、though
C、as
D、as soon as
【正确答案】：C
【题目解析】：as 这里做由于的意思，句意为：由于我母亲不能按时回来，我只好向玛丽求助。