某中学为探究肉汤变质是由空气中细菌引起的，将烧开的肉汤分装在甲、乙两个圆底烧瓶内，实验如图所示．问：

（1）实验探究的问题是______．

（2）甲瓶与乙瓶对照，加上棉塞的目的是______．

（3）预测实验结果：______瓶中肉汤不容易变质．

（4）根据预测结果得出的结论是：______．

根据句意，写出括号内所给单词的适当形式。

小题1:Listening to music or visiting interesting places can make you forget your          (sad).

小题2:As we all know China          (successful) sent up her first man-made satellite in 1970.

小题3:It's very          ( polite) that she left without any words.

小题4:The         (angry) the mother is, the more scared the children become.