people可以加s吗？表什么意思的时候可以加呢？

Copper was the first (metal used) bu humans and is second (only) to iron (into) its utility through (the ages)

答案是C，应该改成什么呢？D是什么意思？.望指教。

C改in D年代、时代。

涉及具体单词用法的问题必须学会查字典。