几道去年这个时候的JJ,概率题,高手帮忙解答一下,谢谢

亡灵一族

Suppose the scores on an examination are normally distributed with a mean of 50 and a standard deviation of 10

<1> what is the probability that the score of a student will be higher than 56.5 ?

A 0.2578   B 0.7422   C 0.7578   D 0.2422   E 0.65

<2>  what proportion of the students scores below 45 ?

A 0.1915   B 0.6915  C 0.50  D 0.3085  E 1

<3> if repeated samples of size  n= 25 is taken from the scores, what is the standard deviation of the distribution of the sample mean?

A 10   B 4   C 0.4    D 2  E none of these