实验室利用过氧化氢、氯酸钾、高锰酸钾等物质制取氧气，其原因是（   ）

- Our head teacher is responsible for his work. He spends most of his time with us.

- ________ He always communicates with our parents in his free time.

两个实习生每人加工一个零件，加工为一等品的概率分别为$\frac{2}{3}$和$\frac{3}{4}$，两个零件是否加工为一等品相互独立，则这两个零件中恰好有一个一等品的概率为（   ）