如图中属于费力杠杆的是（  ）

如图所示，在x轴上方存在着垂直于纸面向里、磁感应强度为B的匀强磁场。一个不计重力的带电粒子从坐标原点O处以速度v进入磁场，粒子进入磁场时的速度方向垂直于磁场且与x轴正方向成120°角，若粒子穿过y轴正半轴后在磁场中到x轴的最大距离为a，则该粒子的比荷及所带电荷的正负是（    ）

下图是出现在20世纪初的《求婚尺牍》，其中有这样的描写：“一不尚貌美，但需端正清白；二需先经几度口头谈话，半年或一年，以朋友资格相处，然后方可订婚；三有知识而无旧式闺秀恶习，且非自命不凡，专尚新时者；态度温和，身体健康；四有牺牲及服务社会之精神。”这反映了当时（   ）

给严重缺氧的病人输氧时，要在纯氧中混入5%的二氧化碳气体，以维持呼吸中枢的兴奋，这属于

阅读下列短文，从每题所给的四个选项中选出最佳选项。

     On a hot summer day, Holmes and Dr. Watson went on a camping trip. After a good meal and a bottle of wine, they lay down in a tent for the night and went to sleep.

     Some hours later, Holmes woke up and pushed his friend. “Watson, look! What can you see in the sky?"

     Watson replied, “I see millions and millions of stars.”

      “What does that tell you?” asked Holmes.

     Watson thought for a while. “First, it tells me that we will have a beautiful clear date tomorrow. Second, it tells me that the time is about twelve o'clock. And…”

      “What else?” Holmes asked.

      “Well, it also reminds me that the world is so big and we are so small. What does that tell you?”

     Holmes said with a grin on his face, “You silly guy! It only tells me that someone has stolen our tent!”