The teacher asked us ________.

A．when did I finish my work

B．why didn’t we tell him about it earlier

C．what we were interested in

D．where we are going to have our lunch

A．条件(1)充分，但条件(2)不充分。

B．条件(2)充分，但条件(1)不充分。

C．条件(1)和(2)单独都不充分，但条件(1)和条件(2)联合起来充分。

D．条件(1)充分，条件(2)也充分。

E．条件(1)和(2)单独都不充分，条件(1)和条件(2)联合起来也不充分。

x²+y²+z²-xy-yz-xz=75。
(1)x-y=5； (2)z-y=10．