未知题型 表面热处理的加热方法主要有（ ）、火焰加热、电接触加热法等。

单项选择题 Read the following noun phrase: those five beautiful shiny Arkansas Black apples sitting on the chair. What is the head noun?

单项选择题 The density of a crystalline polymer will be greater than an amorphous one of the same material and molecular weight because: