A-A+
Why does the author mention the keystone in pa
问题详情
Why does the author mention the keystone in paragraph 1 ？
A．To explain the engineering of an arch
B．To provide historical background on arches
C．To point out one of the virtues of arches
D．To suggest an alternative to the arch请帮忙给出正确答案和分析，谢谢！
参考答案
正确答案：A
解析：... the arch is stable only when it is complete, when the topmost stone, the keystone, has been set in place." Choice B is not correct because the historical background was provided at the beginning of the passage. Choice C is not correct because the virtues are pointed out before the keystone is mentioned. Choice D is not correct because the passage is about arches, and no alternatives are suggested.
