Question

They're going to the park ________.

1. A.
   by bikes
2. B.
   on feet
3. C.
   by bus
4. D.
   in car

Answer 1

C
介词by后面接交通工具名词应用单数形式，同时也不能带不定冠词或其他修饰词，即只能说by car/ bus/ bike不能说by cars/ buses/ bikes或by a car/ a bus，但可以说on a bike/ on bikes in a car，“步行”则是 on foot，故只能选择C。