The river through our city ,which is about ( ),is clean again .A.6000 metres long B.6000-metres-long C.6000-metre-long D.6000 metre long这个我一直不是很明白 有连字符和没有连字符的名词作定语的题 能解释具体一点么?说说错的错在哪了?正确的该怎么用?尤其是a和c的区别~

The river through our city ,which is about ( ),is clean again .
A.6000 metres long B.6000-metres-long
C.6000-metre-long D.6000 metre long
这个我一直不是很明白 有连字符和没有连字符的名词作定语的题 能解释具体一点么?说说错的错在哪了?正确的该怎么用?尤其是a和c的区别~