Which is incorrect about the description of trigone of bladder?() A: It lies at the inner surface of fundus of bladder. B: It is a triangular area formed by two ureteric orifices and an interureteric fold. C: Its submucous layer and muscular layer are connected compactly with each other. D: It maintains smooth all the time no matter extending or contraction. E: It is the predilection site of tumor, concretion and inflammation.