Usually a space is left between the rails of a railway line .( ) one piece

题目

Usually a space is left between the rails of a railway line .( ) one piece
usually a space is left between the rails of a railway line ( )one piece joins the next.
a when b where c so that d as
及其翻译

答案

A;
意思：在架一个接一个的铁轨的时候,两个铁轨中间要留一定的空隙；
排除法比较容易；
文中已经有地点(between the rails of a railway line), 所以不用where
c表示 以便,所以.. d表示 作为...
排除掉 bcd,语境都不合适；