在光纤通讯中，电磁波的传播速度是____m/s，通讯用的激光从光导纤维的一端射入，在内壁上经过多次____（选填“反射”或“折射”），从另一端射出，从而把它携带的信息传到了远方．

孔明灯

我国汉代曾发明过一种用做军事信号的“孔明灯”，它其实就是一种灯笼．先用很轻的竹篾扎成框架，再用纸糊严，并在下面留口．在灯笼的下面固定一个小碟，在碟内放入燃料．点燃后，当灯笼内的空气被加热到一定温度时，灯笼就能腾空而起．灯笼受到的空气浮力可以用阿基米德原理计算，即F_浮=ρ_空气gV （本题g取10N/kg）．温度为20℃时，空气密度为1.2kg/m³，下表给出了一些空气密度和温度的对应关系．

依次填人下面横线处的词语，最恰当的一项是（    ）

①北票翼龙化石创造了多个“世界之最”，已成为解开大自然        的“天窗”和世界关注的焦点。

②我市选派的12名武术健儿在刚刚        的“中国•辽宁国际武术交流大赛”中，斩获5金、14银、3铜。

③我们      要确立好数字化阅读的管理机制，     要筑牢事后监管的红线，通过动态管理，净化数字化阅读的环境。

④构建中美新型大国关系，秉承大国担当、肩负起应有的历史责任，中美就一定能够        两国乃至世界的美好未来。

    Friendship is the most important thing in the world. But how do you find a real friendship and keep it?

    An American writer tells young students some smart ways to find friends. He says finding friendship is just like planting a tree. You plant the seed(种子) and take care of(照顾) it to make it grow(生长).

    First, to make friends, you must be kind and friendly to others. You should learn to smile at others and try to make a stranger(陌生人) feel at home. You should think more of others than of yourself.

    Second, friends should negotiate(商量). When you don't agree with someone, please discuss with him or her. But even the best friends can fight. After the fight, you have to talk to him or her. If he or she doesn't want to talk, you could write a letter. Tell him or her how you are feeling, and tell him or her why you did this or that.

    Finally, if he or she leaves you when you are in trouble, never believe in him or her. And never leave your friend when he or she is in trouble. Remember, a friend in need is a friend indeed.