.

Though the teacher had tried to make the problem easy__________, still Lily and her partner were at a loss.

A．understood

B．understanding

C．to understand

D．to be understood

为预防结石复发，下列需要酸化尿液的结石有（）。

A.胱氨酸结石

B.尿酸盐结石和磷酸盐结石

C.尿酸结石

D.草酸盐结石

E.黄嘌呤结石