请教几道prep里面的DS题

1、A certain list consists of severeal different integers, Is the product of all the intergers in the list positive?
(1)The product of the greatest and smallest of the integers in the list is positive
(2)There is an even number of integers in the list
答案为什么是3呢  我选的是1

2、If ab≠o and points (-a, b)and(-b, a) are in the same quadrant of the xy-plane, is point(-x, y) in this same quadrant?
(1)xy＞0
(2)ax＞0
答案为什么是3呢。。

3、The lifetimes of all the batteries produced by a certain company in a year have a distribution that is symmetric about the mean m. If the distribution has a standard deviation of d, what percent of the distribution is greater than m+d?
(1)68 percent of the distribution lies in the interval from m-d to m+d, inclusive
(2)16 perceng of the distribution is less than m-d
答案是4  对题目的意思都不是很明白，但是感觉是3

4 If 500 is the multiple of 100 that is closest to x and 400 is the multiple of 100 that is closest to y, which multiple of 100 is closet to x+y?
(1)x﹤
500
()y﹤
400
答案是5  不明白题目的意思

请各位路过的大虾帮忙，不胜感激~