the rivers will become dirtier and dirtier --- we take action to protect thema\ sinceb\ ifc\ untild\ unless说理由!

the rivers will become dirtier and dirtier --- we take action to protect thema\ sinceb\ ifc\ untild\ unless说理由!
the rivers will become dirtier and dirtier --- we take action to protect them
a\ since
b\ if
c\ until
d\ unless
说理由!

the rivers will become dirtier and dirtier --- we take action to protect thema\ sinceb\ ifc\ untild\ unless说理由!
D
河水会越来越脏,除非我们采取措施保护他们

D 除非我们采取行动去保护它否则河会越来越脏

除非我们采取措施保护他们