小柯想当班委,因为可以锻炼自己的工作能力,但是想到会占用学习时间和娱乐时间,就很犹豫,她想,有哪种选择是不需要付出代价的呢?你认为(　　)

阅读理解

　  Tim and Li Fang are not going to have any classes next week. They're going to work on a farm.

      They are going to leave at about seven thirty on Monday morning. They are going to travel to the farm by bus. They are going to wear old clothes because they are going to grow rice. They are going to have fish for lunch. They are also going to have a swim at the farm.

     Tim and Li Fang are excited about going to the farm. Not only are they going to help around the farm, but also they are going to learn about farming.

      Li Fang is worried that Tim won't arrive on time to catch the bus because he is nearly always late. Li Fang often has to say to him, “Don't be late, Tim. ”

用图甲所示的装置测出凸透镜的焦距，并“探究凸透镜成像规律”，当蜡烛、透镜、光屏位置如图乙时，在光屏上可成清晰的像．下列说法正确的是（　　）

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