下列物质转化关系中，需要加入适当氧化剂的是（　　）

A．PCl₃→PCl₅

B．KMnO₄→Mn²⁺

C．SO₃→SO₄^2-

D．CO₂→CO

 By the time he was twelve he ______ a chemistry lab of his own.

A．built

B．has built

C．had built

D．was building