Which signal is recognized as a distress signal?（）

A.Directing the beam of a searchlight at another vessel

B.A smoke signal giving off orange colored smoke

C.A whistle signal of one prolonged and three short blasts

D.International Code Signal "PAN" spoken over the radiotelephone

下列关于数据库的密码论述中错误的是( )

A．维护数据库的安全性

B．维护数据库的可靠性

C．必须以独占方式打开数据库才能设置密码

D．撤销数据库密码也需要知道原来设置的密码