Ian Uses a Quickie Manual Wheelchair with a Planar Seat

Ian uses a Quickie manual wheelchair with a planar seat cushion. He extends very hard out of his seating system, pushing up against the abductor (medial thigh) support. His mom says that even with the seat belt Ian will not stay back in the seat unless this abductor support is secured onto the chair. The most likely cause of Ian's positioning problem is

A) the abductor support/pommel is not working effectively and should be tighter and more secure.
B) a lack of sufficient lateral trunk support.
C) the seat cushion is not providing sufficient contour or firmness to give the best posterior support for his pelvis.
D) a lack of sufficient anterior trunk support.