①实验室用石灰石和稀硫酸制备二氧化碳时，收满二氧化碳的集气瓶应正立在桌面上

②实验室用锌和稀硫酸制备氢气时，收满氢气的集气瓶应倒立在桌面上

③点燃任何可燃性气体前，必须先验纯

④制备任何气体时，都要把反应装置中的空气排尽后，再用排水法收集．

如图甲所示，当开关S从点2转到1时，电流表和电压表对应的示数如图乙所示，由图甲和图乙中的信息可知，电源电压是____ V，电阻R₂的阻值是____ Ω，电阻R₂的I－U图像是丙图中的____ ．（选填“a”或“b”）

    Mrs. Green doesn't like sports. She has much work to do at home. Her mother is coming this afternoon and she goes to meet her. Near the school she looks at her watch. She has some time. So she goes into the school. Her son isn't in the classroom. She asks a boy and knows they're behind the school. She sees some boys playing there. Her son is there too. She watches them playing and soon she's angry. She takes her son to the wall. “Go home with me, Richard.” She says, "Don't play with them. I can buy a nice ball for you. Then you don't need to scramble for the ball."

三个相同的数相加的和，等于这个数的____倍。