题目

牦牛的毛色中，黑色对红色为显性。为了确定一头黑色母牛是否为纯合体，应选用的公牛是         A.黑色杂合体       B.黑色纯合体    C.红色杂合体     D.红色纯合体   答案：D Lady Gaga has put off her concerts because of the difficulties ______ by the ongoing strikes.    A. causing       B. to be caused     C. caused     D. to cause