Early this morning we were waiting eager at the school gate for two American guests．They were from an university in New York．All of them were going to give us a lecture．At half past seven，Mr.．Jackson，along by his assistant，arrived．As long as their car stopped，we all went up and said hello to them．Then followed our headmaster，they came to the lecture room in there they gave us a lecture on “American Culture”and another one on “American Senior High School”．They spoke slowly，clearly and loudly，so they could understand most of the lectures．The lectures had interested us a lot．Before that we had a free talk with them and many questions．

枸橼酸抑制草酸钙结石形成的机制是（）

A.增加尿pH值

B.降低尿镁水平

C.增加尿草酸钠饱和度

D.结合尿钙

E.降低尿pH值