题目

In the mid 1990’s, people started doing business on the Internet. At that time, there were two kinds of companies(公司). First, there were traditional companies. They sold things in stores. Then there were Internet companies. They didn’t have a store, and they only sold things on the Net.    Traditional companies didn’t want to lose any business. Quickly they made their own websites and began selling things on the Net. These are the so-called “brick and click” companies. Many stores are made of brick. And you click on your mouse to buy things with your computer. That’s where th如图（a）所示是一种新式手电筒，当沿图中箭头方向来回摇动时，灯泡就台能发光，图（b）四个实验中能解释上述现象的是