According to classification, the difficulty is easy. But I am not sure about that. The question is tricky. I think that the best approach is to dismiss the other options. For example, C and D are out of the scope, because neither other planets nor synthetic materials are mentioned in the passage.

The argument of the passage is: [quote the quantity of a natural resource such as copper cannot be calculated even in principle, because copper can be made from other metals.[/quote]

Option A and E talk about the known and unknown copper deposits but there are not indicated any about to make copper from other methal. Option B is the only that mentioned something about other metals, but its approach about price is something different of the argument but anyway, the answer is this option.