The store had to ____ a number of clerks because sales were down .

A．layout

B．layoff

C．layaside

D．laydown

燃料燃烧实际释放出的总热量与燃料所能释放的总热量之比称为（）。

A.循环热效率

B.空燃比

C.燃烧效率

D.比热