有一读者问： 有这样一道选择题：
(1)The small river runs_________the forest quietly.
A. in B. across C. through D. on
应选B还是C? 有人说across指“横过”，through指“通过”，方向垂直。但词典中却有这样一例：
(2)The river flows from west to east through the city.
不知作何解释。
介词across的确常意指“横过”，一般强调从一边横向到另一边，遂即停止，如横过一条街、一条河等，实例如：
(3)He went across the street. （强调过街为止，至于过街后如何，那就是另外一回事了）
(4)There is a bridge across the river.
介词through的本义是“穿过”或“通过”，可意为直穿过，也可意为横穿过，也可意为斜穿过，穿过之后一般并不就此停止，上述句（2）即是一例。再如：
(5)We drove through the town.
(6)The rain poured through a hole in the roof.
由此可见，上述句（1）应选用through。
