设两野共面变角照射，夹角为θ，为使靶区剂量分布均匀，楔形板的楔形角a应为()

A.a=θ/2

B.tga=k×tgθ(Fw为楔形因子)

C.a=θ×Fw

D.a=180-θ/2

E.a=90-θ/2