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．

甲乙签订保证协议，甲向乙保证：当乙不能偿还对丙的债务时，甲负有向丙偿还该债务的义务，现法院调查：乙丙之间的合同无效，那甲乙之间的保证协议应如何认定( )

A．无效

B．效力待定

C．有效

D．依当事人意愿确定是否有效