选B
这里正确的形式是AB.A是用来做谓语的.例如
In South Korea ,most students take a subway to( get to) school
CD 不是正确的形式,因该是by subway,不用冠词,不加s.
In South Korea ,most students get to school by subway
补充：
乘船去学校一定比乘公交车上学有趣的多.Going to school by boat is much more interesting than by bus.