在越来越多的同龄人选择出国游学、参加精英训练营时,江苏省泗洪县淮北中学的21名高一学生和1名老师却在2013年暑假选择了一次“苦旅”:用40天的时间,“铁流千里”考察队用自己的双脚穿越了4个省份的40多个县、市,每天至少要走30千米,徒步行走1 290千米。2013年8月15日,他们终于到达此行的终点——北京。“铁流千里”考察队的行为启示我们,磨砺坚强意志要　(　　)

阅读理解

     Mr. Lee was in bed and was trying to go to sleep when he heard the bell ring. He turned on the light and looked at his clock. It was twelve o'clock. "Who can it be at this time of night?” He thought. He decided to go and find out. So he got up, put on his dressing gown(长袍) and went to the door. When he opened the door, there was nobody there. "That is very strange." Then he went back to his bedroom, took off his dressing gown, got back into bed, turned off the light and tried to go to sleep.

     A few minutes later he heard the bell ring again. Mr. Lee jumped out of bed very quickly and rushed to the door. He opened it, but again he found no one there. He closed the door and tried not to feel angry. Then he saw a piece of paper on the floor. He picked it up. There were some words on it: "It is now after midnight (午夜), so it is April Fool's Day(愚人节). April fool to you!”

    “Oh, it was the English boy next door!" Mr. Lee exclaimed（惊叫）and almost smiled. He went back to bed and fell asleep at once. The bell did not ring again.

声音是由发声体的____ 而产生的，声音要靠____ 传播；真空____ （填“能”或“不能”）传声。