在高压加热器上设臵空气管的作用（）
参考答案：及时排除加热蒸汽中含有的不凝结气体，增强传热效果
计算题
1. What percentage of area is shaded?
Answer: _ .
2. If a group of 6 people have an average age of 21, and each person is half the age of the next person,
what are the ages of the 6 people?
现在所见的最早的成熟文字出现于[ ]
A．原始社会末期
B．夏朝
C．商朝
D．西周时期
