问题补充：
The river is .
A. fifty meter long
B. fifty-meter-long
C. fifty meters long
D. fifty-meters-long
答案：
A. fifty meter long
B. fifty-meter-long
(答案→)C. fifty meters long
D. fifty-meters-long
解析：句意：这条河五十米长。fifty-meter-long 五十米长的，复合形容词，修饰名词。50米，故用复数，所以选C。
