世界上平均海拔最高、被称为“冰雪大陆”的大洲是[ ]
A、南美洲
B、南极洲
C、亚洲
D、非洲
答案：B
I went to Sanya with my family this summer and all of us _____ a good trip.
A．have
B．are having
C．will have
D．had
何谓溶源细菌的诱导作用？试说明其诱导作用的机理。
