The wealthy are always wealthy and the poor are always poor. It is extremely difficult for the poor _____wealthy, _____ they struggle to their full abilities.    [ ]

A. becoming; even if  

B. to become; even so    

D. becoming; if so  

D. to become; even though

蒸发器中的水，要经常保持清洁，应随时捞取漂浮物，发现器内水体变色、有味或器壁上出现青苔时，即应（）。

A、糖水

B、淡水

C、盐水

D、换水