A、They were possibly caused by man. B、They were mostly caused by volcanoes. C、They were certainly caused by nature. D、We know no

(16)We don’t know how the first fire was made. Early fires on the earth were certainly caused by nature, not by man. Some were caused by lightning in a storm; others, perhaps by one hot material which came out of a volcano. Quite possibly, at times, the heat of the sun set light to some dry grass or leaves. (17)At first, man, like other animals, was probably afraid of fire. He saw that fire could destroy a forest; he knew that fire could hurt his body. So great was the power of fire that he feared it and worshipped it. Gradually, however, with his better powers of thinking, he overcame his fear. Probably he overcame most of his fear when he discovered how to make fire for himself. Throughout the ages he has learned more about fire, how to control it, and how to use it in many ways. Now fire is no longer a master or a god, it is a servant. Again, at some early date, man found how fire could be used to make certain metals from rocks. (18)Some time later, he found out how to make the metal which we now call bronze. This is a mixture of copper and tin. Bronze was a very useful metal. It was hard and tough, but he could shape it by hammering (probably with heavy stones). He could make a tool with a sharp edge for cutting. It isn’t so easy to get iron from its ore and this metal was probably not discovered until very much later.
16. What do we know about early fires on the earth?
17. What was probably man’s response to the first fire?
18. What is bronze made of?