题目内容

Which of the following modes of heat transfer does NOT require any physical contact between a warmer and a cooler substance？

A. Radiation
B. Conduction
C. Natural convection
D. contact directly

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活塞销的主要损坏形式是_______ I. 磨损，II. 摩擦，III. 擦伤，IV. 腐蚀，V. 裂纹

A. I+II+V
B. I+V
C. II+V
D. I+III+V

一个经济衰退的标准定义是实际GDP的下降持续至少两________。

A. 年
B. 季度
C. 月份
D. 星期
E. 参考期间

在图19—2中，经济在何时经历经济衰退？

what chemicals did you use to treat the boiler water ？when you test alkalinity and acidity of water in boiler by pH test paper ？ the result shows the boiler water is in acidiity . please describe what the possible cause is .