[求助] question of problem solving

In OG- quantitative Review(the green book), I don't understand well with the one:No. 172 of problem solving. Below is the question.

If x,y and z are positive integers such that x is a factor of y , and x is a multiple of  z , which of the following is NOT necessarily an integer?

A  (x+z) / z
B  (y+z) / x
C  (x+y) / z
D   xy / z
C   yz / x

In this question, "is NOT necessarily an integer" is most confused by me.I could't understand much about  the  explanation for this question in this book,too.
What does "not necessarily" mean here?How to understand it in a easy way?

Need opinions from you guys~~~
Thank you in advance!

大家帮忙看看好吗？

是B

请详细说明下怎么理解

把语言描述变换成算式，即：x=kz, y=lx，其中k和l是integer。

则只有(y+z)/x=y/x+z/x=l+1/k，只有当k=1时候才成立，所以not necessarily an integer。

其他的选项都能用l或者k表示，你自己算吧。