--Sit down please..A. Thank you.B. Good morning. C. Hello. D. Goodbye

问题补充：

--Sit down, please. --.

A. Thank you.

B. Good morning.

C. Hello.

D. Goodbye

答案：

(答案→)A. Thank you.

B. Good morning.

C. Hello.

D. Goodbye

解析：A.谢谢; B.早上好; C.你好; D.再见.句意: —请坐下。—谢谢。别人说坐下时，自己应该说谢谢。结合语境可知选A。