下列选项中属于跆拳道比赛中允许攻击的部位的是()
A.后脑
B.颈椎
C.肋部
D.大腿
参考答案:C
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