She stared at the painting, wondering where she it.
A:saw
B: has seen
C: sees
D: had seen
若m≤0或n≤0,则m+n≤0,写出其逆命题、否命题、逆否命题,同时指出它们的真假.
生活中我们用凸透镜观察到实像一定是 ____的,虚像一定是 ____的(填写“倒立”或“正立”).
若x,y为实数,且满足|x﹣3|+y-3=0,则(xy)2012的值是____