先通读下面的短文，掌握其大意，然后从A、B、C、D四个选项中选择可以填入空白处的最佳答案．

    One night I saw something on the news that made me sad．It was1a group of homeless(无家可归的)people sleeping outside in the cold．

    So I decided to start a2to raise many for the homeless．I called it the Rock Club,At the beginning,there were only five members in it，3that quickly grew to about twenty．I didn't have to ask anyone if they wanted to be a4．In fact，they came up to me and asked me if they could5the club!

    We spent all of our free time on rock painting．We painted animals,flowers and even the names of sports teams on our rocks．We'd go around the school to6people who would buy them．By Christmas，we had raised thirty．three dollars and then we gave the money to a homeless shelter(收容所)in town．We all felt7that we could do something to help the homeless．

    A few days later,a newspaper reporter(记者)came to our school to8with us about our club and took a photo of us．The photo and article(文章) 9in the newspaper the next day．Later,10school,joined by three other schools，started a project for kids to help homeless shelters．

    Something as simple(简单的)as some painted rocks and a few caring kids made me know that I don't need much to make a difference．

如图是某绿色开花植物的形态结构及生理变化示意图，请据图回：

有一横截面积为S的铝导线，当有电压加在该导线上时，导线中的电流强度为I．设每单位体积的导线中有n个自由电子，电子电量为e，此时电子定向移动的速度为v，则在△t时间内，通过导体横截面的自由电子数目可表示为（   ）