As      rule, apples are sold by      weight and eggs by      dozen.  [ ]

A. a; a; the                

B. a; the; the  

C. a; 不填; the            

D. the; 不填; 不填

A ship must have a self-contained breathing apparatus to be used as protection from gas leaking from a refrigeration unit. To meet this requirement，you may use（）.

A.a gas mask certified by the Mine Safety and Health Administration

B.the same self-contained breathing apparatus required with the fireman's outfit

C.an oxygen breathing apparatus，provided that the device has been inspected within three years

D.a portable ventilation system that will provide a complete change of air every three minutes