melting iron ore. producing lumber. molding plastic. crushing rock into gravel.
I gave my students this multiple choice question: Use the information below to answer the question. Manufacturing uses many steps to change natural resources into products. Some of these steps change natural resources into industrial materials. These steps are called primary processes. All of the following are primary processes except

Only one answer is correct among these 4 choices:
- melting iron ore.
- producing lumber.
- molding plastic.
- crushing rock into gravel.

Could you tell me which one is correct?

molding plastic.