We were having a meeting ______ suddenly someone knocked at the door. [ ]

A. because

B. when

C. as

D. while

复发性口疮黏膜损害的临床特征是（）。

A.散在的圆形或椭圆形痛性溃疡

B.散在的多形性溃疡

C.分散簇集的多数针头大小，透明小疱疹

D.呈白色小丘疹连成的线条或网状损害

E.增生性菜花状溃疡