题目

A couple of years ago, I came to a lady’s house to buy some vitamins. Upon entering the house, I    21   that there was an electronic keyboard on a stand, leaning    22    the wall. Being a piano music lover and  player , I asked if the woman played. She said “Yes” and  23  that she had been taking lessons—at age 54!      “I have been   24   for 8 years now,” I said.  “Then you must play a song for me before you leave,” she  25   .  I   thought she was kidding and I  26  smiled. But by the end of the  purchase  of the vitamins, she�某夫妇具有2种独立遗传的致病基因。已知该夫妇生一个患甲种遗传病的子代的概率是a，生一个患乙种遗传病的概率是b。则该夫妇生一个正常子代的概率是 A.(1－a)+(1－b)　　    B.(1－a)(1－b) C.1－ab　　　　  D.1－a－b－ab