答案：

　　31.由原文“ Nobody knows for sure how people began to use fire. “可知，人类是如何开始使用火的，人们都丌确定。故选 D。

　　32.由原文”But matches can be very dangerous.“可知，火柴十分危险。故选 C。

　　33.由原文”Fires need oxygen (氧气). Without oxygen they will die.This keeps the air away from a fire and kills it.“可知，要扑灭火应使其不空气隔绝，故选 B。

　　34.由原文”Be careful with fire， and it will help you. Be careless with fire， and it will bum you.“可知，若丌当心火可能会烧伤人类，故选 C。

　　35.通读全文，第一段主要介绍火的好处，第二段主要介绍人类用火的起源，第三段主要介绍火的弊端，以及失火的应对措施。由文章开头” Fire can help people in many ways. But it can be very dangerous.“结尾”Be careful with fire， and it will help you. Be careless with fire， and it will bum you.“可知选 B。