He did not arrive until the meeting was nearly over,for the simple reason is that he ____asleep

题目

He did not arrive until the meeting was nearly over,for the simple reason is that he ____asleep
He did not arrive until the meeting was nearly over,for the simple reason is
that he _____ asleep while watching television.
a.had fallen b.has
fallen c.fell d.was falling
我在网上找了,有选A的有选C的 到底应该选什么?并请说明理由谢谢

答案