9.—You didn’t hear me until now? 　　—No. I must ___ soundly w

9.—You didn’t hear me until now? 　　—No. I must ___ soundly when you came in.
9.—You didn’t hear me until now?　　—No. I must __D_ soundly when you came in. 　　A. have slept B. be sleeping C. sleep D. have been sleeping请解释下A项为何不对