1-23.Every few-----along the Great Wallthere are some watch

1-23.Every few_____along the Great Wallthere are some watch towers.
B.hundreds of metres
C.hundred meters.
我觉得2个答案都可,为什么只有C对?并且我认为C中的”meters”不能加”S”,着又是为什么 呢?能详细地一个个解释吗?谢谢啦!