By the time she reached the corner of the street, the footsteps _________. A. had gone B. were gone C. are missing D. have disappeared 答案:B一对夫妇,其后代若仅考虑一种病的得病率,则得病可能性为 a,正常的可能性为 b,若仅考虑另一种病的得病几率,则得病的可能性为 c,正常的可能性为 d.则这对夫妇结婚后,生出只有一种病的孩子的可能性的表达式可表示为( )①ad+bc; ②1-ac-bd; ③a+c-2ac; ④b+d-2bd.
A、①②B、①②③C、②③D、①②③④