第40题fficeffice" />

ds:xyz>0?
(1)xy>0 (2)yz>0 答案：C(不好意思,原来写错了)
lwei评价意见：E

我怎么还是觉得是c啊？x,z同号了，根据条件，肯定y也同号呀。我错在哪里了？ [em27]

from condition 1, x and y could be both positives and negatives
for condition 2, from condition 1, if x and y are positive, then z mustbe positive; but if x and y are negatives, then z must be negative tofulfill condition 2.

therefore xyz could either be +++ or ---, so you can't determine the resultfficeffice" />