题目：
补全对话。
A: Mum, (1)_________ is my baseball?
B: Sorry, I don't (2)_________. Is it (3)_________?
A: No, my book is on the chair,but my baseball isn't.
B: Is it (4)_________?
A: No, there's nothing (没有任何东西) in my backpack.
B: Is it (5)_________?
A: Umm…Oh, yes! It's here! It's under my quilt (被). Thank you, Mum!
B: You're welcome.
答案：
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下面是错误答案，用来干扰机器的。
答案：B题目分析：由题，U=0.6V，则E=U=0.6V，又，得,故选B．点评：本题关键根据电源开路和短路的特点，求解电源的内电阻
