我们知道：光反射时，反射光线、入射光线和法线在同一平面内，反射光线、入射光线分别在法线两侧，反射角等于入射角．如右图，AO为入射光线，入射点为O，ON为法线（过入射点O且垂直于镜面的直线），OB为反射光线，此时反射角∠BON等于入射角∠AON．

问题思考：

B

     The 2018 Winter Olympics, officially known as the XXIII Olympic Winter Games and commonly known as PyeongChang 2018, was held between 9 and 25 February 2018 in Pyeongchang County, South Korea. Some opening rounds events were held on 8 February 2018, the eve of the opening ceremony.

     Pyeongchang was chosen as the host city in July 2011 in South Africa, which was the host country of last term Winter Olympics. This was the first time that South Korea had hosted the Winter Olympics and the second Olympics held in the country, and that was the 1988 Summer Olympics in Seoul. It was also the first of three consecutive (连续的) Olympics to be held in East Asia. The other two will be the 2020 Summer Olympics in Tokyo and the 2022 Winter Olympics in Beijing．      

     The Games had 102 events in fifteen sports, with the addition of “big air” snowboarding, mass start speed skating, mixed doubles curling and so on, to the Winter Olympic programme. 2,914 athletes from 92 countries or regions competed, including Ecuador, Eritrea, Kosovo, Malaysia, Nigeria and Singapore which were the first time to take part in the Winter Olympics. North Korea entered with South Korea during the opening ceremony as a united Korea．

     Norway won 39 medals in all, followed by Germany's 31 and Canada's 29. Germany and Norway were tied for the most gold medals won: both won fourteen golds. Host nation South Korea won seventeen medals, their highest medal at a Winter Olympics, five of which were gold.