背部疼痛可发生于

A．胰腺癌

B．球后溃疡

C．穿透性溃疡

D．幽门梗阻

E．老年人消化性溃疡

（）we stand away from the church bell, it takes time for the sound waves to each us.

A.If

B.Whenever

C.Because

D.Where