A:O点的电势最低
B:x2点的电势最高
C:x1和-x1两点的电势相等
D:x1和x3两点的电势相等
—How long have you _________ your watch? It looks new.
—Only about three weeks.
设fx=x3+log2x+x2+1,则对任意实数a,b,a+b≥0是fa+fb≥0的( )