the rivers will become dirtier and dirtier --- we take action to protect thema\ sinceb\ ifc\ untild\ unless说理由!
问题描述：
the rivers will become dirtier and dirtier --- we take action to protect them
a\ since
b\ if
c\ until
d\ unless
说理由!
答
除非我们采取措施保护他们
答
D 除非我们采取行动去保护它否则河会越来越脏
答
D
河水会越来越脏,除非我们采取措施保护他们