Participants of Rushton Et Al

Participants of Rushton et al. walked in a curved path towards a target when fitted with prisms which moved the visual scene sideways. The authors suggested that this was because:

A) Optic flow was distorted by the prisms
B) Visual direction, but not optic flow, was changed by the prisms
C) Walking correctly was harder when the visual field was constricted
D) The prisms impaired stereoscopic vision