按我的语感是这样理解的。through 表示穿过,deep valleys 是狭窄地带,当然是穿过了,河流流经的国家,也可以用through. 平原或高原总体是个平面的感觉,所以用across
在人教版必修一的课文中,既有Look at the map on page 18 and list the countries
that the Mekong River flows through.又有At first the river is small
and ... It becomes rapids as it passes through deep valleys, travelling across
western Yunnan Province.这样的句子。请问是与river搭配的时候through和across都可以吗?还是从什么角度做选择?