lgx，lgy，lgz成等差数列是由y²=zx成立的（　　）

A．充分非必要条件

B．必要非充分条件

C．充要条件

D．既不充分也不必要条件

- So, can you tell me ______ here today?

- Well, I was walking down Centre Street when a UFO landed.

A．what did you see

B．what you saw

C．when did you see it

D．when you saw