Which (if any) of the following functions represent constant elasticity of demand (CED) curves? (Consider only the range where quantity demanded Qd and price P are positive.)

1) I did not understand the emphasis and means of range where q and p are positive what is the point?

2) please explain what is ced curves and expand on it

3) why is c and d the only correct answers?

B Q = P2 - 5P + 100 C Q = 15/p5 D Q = 60/5P