The difference between Fixed pitch propeller (FPP) and Controllable pitch propeller (CPP) is in that（）

A.for FP, once fitted, the position of the blades can also be changed in operation
B.for CP, once fitted, the position of the blades can also be changed in operation
C.the CP type has a relatively smaller hub compared with the FP propellers
D.the FP type has a relatively larger hub compared with the CP propellers