沿海地区出现的“赤潮”从种群数量变动角度看是属于（）。

A．季节性消长

B．不规则波动

C．周期性波动

D．种群的爆发

On a ship，a door that is required to be marked KEEP CLOSED is designed to（）.

A.prevent the passage of flammable gases

B.prevent the passage of poisonous vapors

C.delay the spread of heat and flames

D.maintain watertight integrity