It can be inferred from the last paragraph that scientists( ). A.arepuzzledbythem

It can be inferred from the last paragraph that scientists（）.

A．are puzzled by the mystery of forest pollution

B．feel relieved by the use of unleaded gasoline

C．still consider lead pollution a problem

D．lack sufficient means to combat lead pollution

请帮忙给出正确答案和分析，谢谢！