若n为正整数,且x2n=7,则(3x3n)2-4(x2)2n的值为[ ]
A.833
B.2891
C.3283
D.1225
答案:B
A distribution with a mean of 15 and a range of 2 to 25 would be:()
A. normally distributed.
B. positively skewed to the right.
C. cannot be determine.
健康教育和健康促进的目的是()。
A.开展健康传播
B.制定健康的公共政策
C.实施健康干预
D.形成有益于健康的行为
E.提供物质环境支持