“石油企业自行下调价格有利于提高销售量”，对此认识正确的是　　[     ]

A．价格变动会影响需求

B．要提高销售量必须降价　　

C．是价值规律自发调节的结果

D．需求会影响价格

Smallpox, a kind of disease, has now died ________.

A．out

B．away

C．off

D．down