请将①②处的内容补充完整。

如图，四边形ABCD是菱形，∠DAB=50°，对角线AC，BD相交于点O，DH⊥AB于H，连接OH，则∠DHO=____ 度．

阅读下面材料，在空白处填入适当的内容（不多于3个单词）或括号内单词的正确形式。

For many years, volunteers, both young and old, have been helping the poor and elderly people of this area. They work long hours building wheelchair paths, fixing leaky(有漏洞的) roofs and repairing the homes of those____ need help. Their only payment is the happy smiles and grateful hearts of the people whom____have helped.

I will always remember one man in particular____he has a real enthusiasm for voluntary work. He seemed to have inspired all the people in his group with his____(cheer) spirit. Every day when I asked him____he was doing, he would reply, “I'm doing good!”____the end of the month as he was preparing to leave, I looked at all the homes he ____(fix) and had to agree that he was indeed doing____good job!

A nice house, a successful career, and a lot of money might be nice to have, but____(compare) with fixing others' home and helping those who are in need, their____(significant) is not so great.

某物理兴趣小组探究平面镜成像的具体过程如下：

（1）取一张白纸，在白纸上画一条直线MN，并在直线一侧标注三个位置，如图；

（2）将白纸平整的放在水平桌面上，在白纸上沿直线放好透明玻璃板；

（3）找两个完全相同的蜡烛，将其中一支点燃，放在1点，通过玻璃就能观察到蜡烛的像，一边观察像，一边将另一支未点燃的蜡烛放置到与像完全重合，然后记下蜡烛的位置，记录点为a；

（4）拿走未点燃的蜡烛，并在相同位置放上光屏，不通过玻璃板直接观察光屏上是否有像；

（5）再将点燃的蜡烛依次放在2、3两个位置，重复（3）、（4），记录点分别为b、c，最后获得如图记录；

（6）将蜡烛和玻璃板取走，分别连接1a、2b、3c，并测量各点到平面镜的距离；

（7）分析数据，获得结论．

请完成下列问题：

【假设猜想】气体可能是HCl、NH₃、H₂的一种或几种．

【查阅资料】

①NH₃+HCl═NH₄Cl    2NH₃+H₂SO₄═（NH₄）SO₄

②2NH₃+3CuO $\underset{\_}{\underset{\_}{△}}$ 3Cu+N₂+3H₂O

【实验探究】