两电阻R1=R2=10Ω，串联在电路中通电时间为t，产生的热量为Q。若将它们并联起来接入电压为原来一半的电路上，通电时间仍为t，产生的热量为Q’，那么Q：Q’为（　　）

Complete the following passage with the words or phrases in the box. Each can be used only once.(将下列单词或词组填入空格，每空格限填一词，每词只能填一次)

 Today I'm going to talk about rainforests. Are you  ____    with them? They're interesting and exciting places! Many kinds of the world's plants and animals live in the rainforests.

 However, rainforests are____ now---they are disappearing very fast because they are cut down or burnt every day. As a result,   ____  animals and plants are dying out.

 Maybe you have never been to a rainforest, so why should you protect them  ____  ?  Rainforests are important to everyone. They are important because the plants in the rainforests make oxygen . We need oxygen to breathe.

如图所示，竖直悬挂一根长5m的铁棒AB，在铁棒的正下方距铁棒下端5m处有一圆管CD，圆管长10m，剪断细线，让铁棒自由下落，则铁棒通过圆管所需的时间为 （g取10m/s²）．（   ）