设A={A，B，C}，B={-1，0，1}，f：A→B是A到B的映射，使得f（a）+f（b）+f（c）=0，这样的映射的个数是 ______．

Jim can you tell me _____ every week?          [ ]

A. when do you have class meetings

B. when you have class meetings

C. when did you have class meetings

D. when you had class meeting