Which of the Following Activities Would Require the Most Force

Which of the following activities would require the most force, and why?

A) Pushing a wheelchair from a stopped position, because it was stopped
B) Pushing a wheelchair from a stopped position, because you need to overcome friction to move it
C) Continuing to push a wheelchair that was already moving, because you would be overcoming gravity to move it
D) Continuing to push a wheelchair that was already moving, because you would be overcoming friction to move it