属DNA病毒的是（）
A．甲型肝炎病毒
B．乙型肝炎病毒
C．丙型肝炎病毒
D．丁型肝炎病毒
E．戊型肝炎病毒
参考答案：B
. He made up his mind to devote his life____________the river____________polluted.
A．to prevent, to be
B．to prevent, from being
C．to preventing, to be
D．to preventing, from being
SQL的全局约束是指基于元组的检查子句和( )。
A．非空值约束
B．局部约束
C．断言
D．外键子句
