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