Many British people have noticed that butterflies _______ from its natural habitats during the past several years.

A．are disappearing

B．have been disappeared

C．have been disappearing

D．will have disappeared

若联系为m:n，如何确定关系码______。

A) 每个实体集的码均是该关系的候选码
B) 关系的码为n端实体集的码
C) 关系的码为m端实体集的码
D) 关系的码为各实体集码的组合