Assuming her other foot wasn't a) in the air or b) also in A's frontcourt, you've got nothing as she hasn't established position in her frontcourt. For situation "A" check out the case book 9.9.1 Sit A, "B" above is covered by 9.9.1 Sit C-a.
Her position is considered backcourt if one foot is in frontcourt and the other in backcourt.