用均匀导线做成的正方形线框边长为0.2 m，正方形的一半放在垂直于纸面向里的匀强磁场中，如图所示。当磁场以10 T/s的变化率增强时，线框中a、b两点间的电势差是

A．U_ab＝0.1 V

B．U_ab＝－0.1 V

C．U_ab＝0.2 V

D．U_ab＝－0.2 V

Smoking does great harm to our health. ( ) is smoking allowed in public places.

A. In no time



B. At a time
C. At all times


D. At no time