答案：

　　31.由原文“ Nobody knows for sure how people began to use fire. “可知，人類是如何開始使用火的，人們都丌確定。故選 D。

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

　　33.由原文”Fires need oxygen (氧氣). Without oxygen they will die.This keeps the air away from a fire and kills it.“可知，要撲滅火應使其不空氣隔絕，故選 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，若丌當心火可能會燒傷人類，故選 C。

　　35.通讀全文，第一段主要介紹火的好處，第二段主要介紹人類用火的起源，第三段主要介紹火的弊端，以及失火的應對措施。由文章開頭” Fire can help people in many ways. But it can be very dangerous.“結尾”Be careful with fire， and it will help you. Be careless with fire， and it will bum you.“可知選 B。