--- How many students are there in the classroom?

--- ______．All of them are practicing _____ the new song in the music hall．

A．Nobody; singing

B．None; singing

C．Twenty; to sing

D．No one; to sing

下列哪种疾病腹部平片不能诊断（）

A.胆囊阳性结石

B.肠梗阻

C.慢性胰腺炎

D.消化道穿孔

E.小儿先天性肛门闭锁