根据句意及首字母或汉语提示完成单词。

1. He wants to go back to a mountain v______ to teach the children after leaving the college.

2. The old man is p______ but happy because he has many friends.

3. The air is t______ in high mountains.

4. The woman was very sad because her ______ (丈夫) died in the accident.

5. He ______ (返回) to the hotel after he finished having the meeting.

等质量的下列烃完全燃烧，生成CO₂和H₂O，耗氧量最多的是（　　）

A．C₂H₆

B．C₃H₈

C．C₄H₁₀

D．C₅H₁₂