— Oh, dear! I forgot my best friend’s birthday.

— ________ You'd better keep it in mind next year.

如图所示，正方形线圈abcd谢绕对称轴OO′在匀强磁场中匀速转动，转速为 $\frac{50}{\pi }r/s$ ，已知ab=ad=20cm，匝数N=100，磁感应强度B=1T，图示位置线圈平面与磁感线平行．闭合回路中线圈的电阻r=4Ω，外电阻R=12Ω．求：

When you see an English film or talk with native speakers, observe their facial expressions and g____.

   Today is Saturday. It’s sunny and warm. Many children from No. 1 Middle School are at the Old People’s Home. Now, they’re helping the old people.
   Look! Li Min and Zhan Ran are cleaning the windows. Ma Lan and Liu Qian are washing clothes.
   What’s Li Na doing over there? Many old people are clapping their hands(鼓掌)for her. Oh,  she’s singing for them. Her song is really beautiful. And where’s the American boy, Tom? He likes helping others and he often comes here. Listen! Who’s speaking English? It’s Tom. He’s teaching two old people to speak English.
   All the children and the old people are very happy. “We are happy when we help others, ” says Tom.