When you hear the word quicksand, what image comes to mind? You probably picture someone up to the waist in wet land, screaming for help. In fact, quicksand can be very dangerous. People and animal
They lived a hard life then, for their father _______ for half a year. A. had lost his work B. has lost his work C. had been out of work D. has been out of work
逆否命题正确;逆命题不正确
否命题不正确.故选B.