题目内容：

请根据语境用括号内单词的正确形式或根据汉语意思填空，每空一词。

1.How many ________ (胃) does a cow have？

2.The best way to success is to stop ________ (regret) for the past or worrying about the future, but to start focusing on what you can do at present.

3.He explained ________ (science) the most important questions of political economy.

4.There are no ________ (永久的) displays in the museum and exhibits change all the time.

5.He looked at his students with great ________ (satisfy).