What, then, was a child to make of someone who didn’t know—who was never taught—the proper way to shake hands? 作什么成分?
破折号和逗号作用差不多,这里的who was never taught是补充解释为什么didn't know. 所以,应该是非限定性的,有没有它不影响对句子的总体理解。此句说的是孩子会如何看待不懂正确握手的人。
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