Compared with last year,our pay

Compared with last year,our pay _____ about $500 every month.
Compared with last year,our pay _____ about ￥500 every month.
A.has increased by\x05\x05\x05\x05\x05B.has increased C.increased by\x05\x05\x05\x05\x05\x05D.increased to
课本是这样的the population is creasing by 80 million every year.但是上一道题是B 而不是A?

问题在by 和every month上.
个人认为：increase by.增加了.本题如选A,则【表明每个月都增加500】,即：如原工资为5000,一月为5500,二月为6000,.
而The population is creasing by 80 million every year是可以的.
仅供参考,欢迎探讨,
再问： 但是答案是B哟，我就不知道 为什么不是A答案呢？
再答： 是选B呀！ 如选了A，就是本题如选A,则【表明每个月都增加500】，即：如原工资为5000，一月为5500，二月为6000，....难道每月工资都会涨500？？？