Fugitive slaves traveled to Pennsylvania using all of the following methods except?

о wagons.

о railroads.

о slave ships.

о walking

Slave ships-orginally slaves where brought to the americas via slave ships

Well, this is very easy, first up; what's a fugitive?

A fugitive is a person that has escaped a place/or is in hiding; to avoid arrest/persecution.

So traveling by wagons, railroads, and walking makes sense. Except for one, slave ships, that makes no sense at all. Why would fugitive slaves travel on "slave ships?"