All the students _______ very hard over the last three years.
A:works
B:has worked
C:have worked
D:worked
设f(x)是R上的奇函数, 且在(0, +∞)上递增, 若f(12)="0," f(log4x)>0, 那么x的取值范围是( )
下列各图中,已标出电流I、磁感应强度B的方向,其中符合安培定则的是( )