实验法是生物学研究的重要方法。在难以直接拿研究对象做实验时，常采用模拟实验的方法。某班学生用黑、白围棋子模拟生殖细胞来探究人类生男生女的几率问题。他们做了如下的模拟实验：

Ⅰ．实验材料：黑棋子10颗，标记X；白棋子10颗，5个标记X、5个标记Y；纸盒两个，代表父亲和母亲。

Ⅱ．实验步骤：

①将10颗黑棋子和10颗白棋子分别放入两个纸盒中，各自混匀。

②随机从两个纸盒中各取一颗棋子，在表格中记录染色体的组合情况。然后将选取的棋子放回原纸盒中，混匀。

③重复步骤②几次。

Ⅲ．某小组实验结果如下表：

次数	1	2	3	4	5	6	7	8	9	10
结果	XY	XY	XX	XY	XX	XY	XX	XY	XX	XY

请结合上述实验回答下列问题：

硝酸铵是一种常用的化肥，受热易分解．在一定条件下发生如下反应：NH₄NO₃ $\underset{\_}{\underset{\_}{一定条件}}$ X↑+2H₂O．上述化学方程式中X的化学式为（   ）

下列生态系统,自动调节能力最弱的是(　　)

    There was a famous church in London. It was 110 meters high. The center of the church was like a bell. The ceiling of the church was painted by a famous artist. A platform （平台）was built so that the artist could put up the ladder on it and paint the ceiling. The platform was about 70 meters over the ground.
    One day the artist was working hard and had nearly finished one popular corner of the ceiling. Also on the platform was his assistant, who was mixing paints. The artist stepped back so that he could see how his work was progressing. To have a better look at his painting, he stepped back again. Suddenly, his assistant shouted, picked up a small bowl of paint and started to paint the artist’s work on the top. The artist was very angry and rushed forwards to stop him.
    “What do you think you are doing?” he shouted angrily. “Are you mad?” His assistant replied, “I saw you walking backwards to have a better look at your painting. But you didn't notice that you had reached the very edge of the platform. You were in great danger. I wanted to make you move forwards. If I had not made you run forwards, you would have fallen backwards off the edge of the platform.

给下面的句子填上合适的关联词语。