▲若语义需要，by bus, by bike, by ship, by plane等表示方式的介词短语都可以用作表语。如：

The quickest means of travel is by plane. 旅行的最快方式是乘坐飞机。

The best way is by air; the next best (way) is by train. 最好的方法是乘飞机，其次是乘火车。

▲比较下面的句子也属类似用法：

Admission is by invitation only. 凭请柬入场。

Payment is by contribution. 付款采用捐赠的方式。

Entrance to the theatre is by ticket only. 凭票进入剧院。

Admission to this school is by examination. 进入该校须经过考试。