下列四条DNA分子中，彼此间具有黏性末端的一组是（）
①②③④
A.①②
B.②③
C.③④
D.②④
参考答案：B
Ray is a new graduate, and it seems that he has difficulty in dealing with ________ problems arise.
A．whichever
B．whenever
C．whatever
D．wherever
②
③
④
②③④