B

    Hi,everyone! My name is Linda.I'm 1 English girl.This is a 2 of my family.You can see 3 people in it.These are my grandparents.4 are in the middle of the photo.The tall(高的)man is my father .5 name is Henry.This is my mother,Anna.My parents 6 three children(孩子),Tom,Paul and me.Tom and Paul are my 7.Tom is seven.Paul is three.And I'm six.8 is the girl?She is my aunt.Her name is Betty.She is my father's 9.Oh,Pipi is in the photo,too.Pipi is the 10 of my dog.I like it very much.It's my good friend.

物理课上，老师做了一个奇妙的“跳环实验”。如图，她把一个带铁芯的线圈L、开关S和电源用导线连接起来后.将一金属套环置于线圈L上，且使铁芯穿过套环。闭合开关S的瞬间，套环立刻跳起。某司学另找来器材再探究此实验。他连接好电路，经重复试验，线圈上的套环均末动。对比老师演示的实验，下列四个选项中,导致套环未动的原因可能是（   ）