食管癌大多是（）。

A．腺癌 

B．鳞状细胞癌 

C．印戒细胞癌 

D．粘液腺癌 

E．小细胞未分化癌

The ARQ mode should be applied（）.

A.when two stations communicate with each other

B.in any radio system and at any time

C.when one ship sends a message to another station

D.for communication among some stations