On the mucous membrane surface of the urinary bladder, which of the following structures can be identified?
A.external urethral orifice (尿道外口) B.ureteric orifices （输尿管口） C.internal urethral orifice (尿道内口) D.trigone of bladder E.interureteric fold (输尿管间襞)正确答案BCDE
