“掩耳盗铃”是大家熟悉的成语故事．从物理学角度分析，盗贼所犯的错误是：既没有阻止声音的____ ，又没有阻止声音的____．

如图所示，真空中有一平行玻璃砖，一束单色光从光源P发出与界面成θ=30°角从ab面中点射入，测得折射角α=45°，在玻璃砖的右侧面ad射出，假设，光从光源P到ab面上的O点的传播时间和它在玻璃砖中传播的时间相等，已知光在真空中的传播速度c=3.0×10⁸m/s，ab边长为 $\sqrt{3}$ ×10cm，求：

①璃砖的折射率n；

②点光源P到O点的距离．

    We are students from Thailand,and we decide____(save) elephants. The elephant is one of Thailand's____(symbol). Our first flag had a white elephant____it. This is a symbol of luck. Elephants are smart animals. It is easy for them to play soccer____music. They can also draw very____(good). In addition, they are strong enough to carry a lot of things for people. People say that an elephant never____(forget). They can walk for a long time and never get____(lose). They can also remember places with food and water. This helps them to live. But now elephants are in great____(dangerous). People cut down many trees, so elephants are losing their homes. People also kill____(they) for their ivory.Today there are only about 3,000 elephants. It is necessary for us to try our best to save the trees and not to buy things that____(make) of ivory. It is important to remember that March 13th is Thai Elephant Day.