it was evening--we reached the little town.A that B until C

C
我们还没到小镇天就黑了.
这不是强调句,因为强调句有一个特点,就是去掉强调结构It is/was...that句子意思及结构仍然完整.这句话如果去掉这个结构就成了：
We reached the little town (in the) evening.也就是少了in the
所以排除A.
如果选B意思不通.“直到我们到达小镇天黑了.”
如果要表达“直到我们到达小镇天才黑”,应该用not.until
It was not evening until we reached.
选C,从字面意思上看是在我们到达小镇前,就已经是晚上了.也就是：我们还没到小镇天就黑了.