集合A={a，b}，B={1，2}，则从集合A到集合B的映射有( )个
4
It is _____that he has been addicted to the drugs and has difficulty quitting it.
A．obviously
B．apparently
C．possibly
D．likely
设一棵树T的度为4，其中度为1，2，3，4的结点个数分别为4，2，1，1。则T中的叶子结点为
A．8
B．7
C．6
D．5
