Iron is harder to craft into weapons and armor due to its melting point and it took a while to develop furnaces that could melt iron; it was a lot easier to melt copper. People used copper tools due to it's low melting point and malleability...so that's why iron tools didn't develop till a later time period.
Iron tools and weapons were developed later than copper tools due to the difficult extraction process and the higher temperatures required for smelting iron. The technology for working with iron took longer to advance compared to copper and bronze techniques. Therefore, the best choice is A. Is more difficult to extract from its ore.
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