肺灌注显像所用显像剂为（）
A.99mTc-MAG
B.99mTc-MAA
C.99mTc-DTPA
D.99mTc-DMSA
E.Kr
参考答案：B
基因工程的原理是“移花接木”，嫁接就是移花接木，也属于基因工程______．
He suddenly fell ill, ______ prevented him from attending his friend’s wedding.
A．why
B．what
C．that
D．which
