单选题 There are 30 students in a class. 12 of them play basketball, 15 of them play football, and 19 of them play pingpang. If 7 of them play all three activities and exactly 6 of them play two activities, how many of them play none of the three activities?
【正确答案】 D
【答案解析】设玩三种运动的人分别为A，B，C，A=12，B=15，C=19，7个人三种运动都玩，那么P=7，6个人玩两种运动，那么m=6，代入公式：30=12+15+19-6-2×7+N，得到N=4，选择D。