According to Harriet Tubman: Conductor on the Underground Railroad, why is Tubman trying to help people escape to Canada instead of the northern United States?

The Fugitive Slave Law is enforced in the northern states but does not exist in Canada.
The Fugitive Slave Law exists in Canada, and this regulation keeps people safe if they escape.
Those traveling with Tubman believe that Canada is preferable, because it is farther away.
Tubman believes that Canada is a more pleasant place to live than the states.