( )She will go there ------ foot and I willgo there ----- a

这个短语均属“介词+交通工具名词”结构.
"by+交通工具名词“用来泛指采用的交通手段,by后的名词前不能带任何限定词.累死的短语还有：by train,by ship,by plane,by bike,by car,by air,by sea,by land等.例如：
Did you travel to school by bike?
你是骑自行车一路观光到学校的吗?
He went by air,and sent his heavy boxes by sea.
他坐飞机去,但重箱子用船运.
”in(on)+交通工具名词“用来表示具体的交通手段,名词前除了用定冠词外,还可根据具体情况用其他限定词（如a,this,that,your,my,etc.）.一般情况下,”in+交通工具名词“与”on+交通工具名词“可以换用,除bike,horseback这类无厢、无舱的用on以外.但在the car前用in最为常见.例如：
Don't read books in (on) the bus.
不要在公交汽车上看书.
I shall have lunch on (in) the train.
我将在火车上吃午饭.