课外兴趣小组活动时，某校两同学通过实验研究小灯泡的电功率跟两端电压的关系，已知小灯泡的额定电压为2.5伏、额定功率为0.75瓦．小明同学设计了如图甲所示的电路进行研究，他先测出若干组电压和电流值，再由公式P=UI，求得对应的功率，并作出功率随电压变化的图线a．小红同学设计了如图乙所示的电路进行研究，她先由R= $\frac{U_{额}{}^2}{P_{额}}$ 计算出小灯泡的电阻R，再测出若干个电压值，最后根据P= $\frac{U^2}{R_{}{}^{}}$ ，求得对应的功率，也作出功率随电压变化的图线b．则下列反映了他们的实验结果的图线是(   )

在研究性学习活动中，几名同学对家用微波炉的实际功率和加热效率进行了研究．下表所示为记录的相关内容．

主要器材	微波炉	电能表
相关资料	额定电压220V 频率50Hz	220V   10（20）A 50Hz   600revs/kW•h
活动记录	①取①质量为250g的水，测得水的初温为20℃ ②关闭家中其它用电器，只将该微波炉接入电路中，让其单独工作 ③用微波炉给水加热1min ④测得电能表的转盘在这1min内转了12转 ⑤加热1min后，水的温度达到60℃

在用微波炉给水加热的1min内，求：

某公司今年年初用25万元引进一种新的设备，投入设备后每年收益为21万元．该公司第n年需要付出设备的维修和工人工资等费用a_n的信息如图．

根据短文理解, 从A、B、C三个选项中选择最佳答案填空。

    Miss Richards was a teacher at a school for boys and girls. She 1 chemistry and physics from the lowest to the highest classes in the 2. Sometimes the new classes 3 rapidly, but sometimes they were very 4, and then Miss Richards had to 5things many times.

    One year, the first class had been studying chemistry for several 6 when Miss Richards suddenly asked, "What is water? Who knows? 7 up!"

    There was silence for a few seconds, and Miss Richards felt sad, but then one boy 8 his hand.

    "Yes, Dick?" said Miss Richards encouragingly. He was not one of the brightest children in the class, so she was 9 that he could answer.

  "Water is a liquid which has no 10 until you wash your hands in it, Miss. Then it turns black, "the boy replied with great confidence.

There ______ many people ______ in the office, but now nobody is allowed to.