关于热机的效率，下列说法中正确的是（ ）
A．热机做的有用功越多，热机效率一定高
B．热机的功率大，热机效率一定高
C．热机单位时间内消耗的燃料少，热机效率一定高
D．以上说法都不对
答案：D
题目分析：热机的效率是指热机用来做有用功的能量与燃料完全燃烧产生的能量之比；热机的效率高，说热机用来做有用功的能量与燃料完全燃烧产生的能量比值高，而不是有用功越多，机械效率越大，A错误；功率是表示物体做功快慢的物理量，热机的效率是指热机用来做有用功的能量与燃料完全燃烧产生的能量之比，两者之间无关，C错误，选D。
点评：解决本题的关键是熟知功率和机械效率的区别：热机的效率是指热机用来做有用功的能量与燃料完全燃烧产生的能量之比；功率是表示物体做功快慢的物理量。
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| Food is important. Everyone needs to 1 well if he or she wants to have a strong body. Our minds also need a kind of food. This kind of food is 2 . We begin to get knowledge even when we are young. Small children are 3 in everything around them. They learn 4 while they are watching and listening. When they are getting older they begin to 5 story books, science books..., anything they like. When they find something new, they have to ask questions and 6 to find out the answers. What is the best 7 to get knowledge? If we learn 8 ourselves, we will get the most knowledge. If we are 9 getting answers from others and don't ask why, we will never learn more and understand 10 . | ||||
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