要考试了，请教一道数学题

A vending machine is designed to dispense 8 ounces of coffee into a cup.  After a test that recorded the number of ounces of coffee in each of 1,000 cups dispensed by the vending machine, the 12 listed amounts, in ounces, were selected from the data.  If the 1,000 recorded amounts have a mean of 8.1 ounces and a standard deviation of 0.3 ounce, how many of the 12 listed amounts are within 1.5 standard deviations of the mean?

(A) Four
(B) Six
(C) Nine
(D) Ten
(E) Eleven
答案是E，请问这道题该怎么做啊？

求求哪位给我讲解一下嘛。。不甚感谢！

哦，这是PREP破解上面的题，难道是题目不全？

谢谢楼上