A marine biologist placed a shark into a large holding tank and then released several small bait fish into the tank. The result met your 1, the shark quickly swam across the tank, attacked and ate the smaller fish.

    The biologist then 2a strong piece of fiberglass into the tank, creating two 3parts. She then put the shark on one side of the 4fiberglass and a new set of bait fish on the other. 5, the shark quickly attacked. But this time, the shark 6because it knocked hard against the fiberglass 7. Determined, the shark kept 8this behavior every few minutes, but 9. Meanwhile, the bait fish swam around 10in the second part. Eventually, the shark 11hope of hunting. This experiment was carried out several times. Each time, the shark got less 12and made fewer attempts to attack the fish, until it 13hitting the fiberglass and simply stopped attacking. The biologist then 14the fiberglass, but the shark didn't attack. The shark was trained to believe a 15existed between it and the bait fish, 16the bait fish could swim wherever they wished, free from harm.

    Many of us, after experiencing setbacks and failures, 17give up and stop trying. Like the shark in the story, we believe that 18we were unsuccessful in the past, we will always be unsuccessful. 19, even when no "real" obstacle exists between where we are and where we want to go, we 20see obstacles in our heads.

图中a、b两点纬度相同，但地球自转的线速度明显不同，原因是（  ）

借助历史时间轴，归纳史实，梳理历史发展线索是学习历史的有效方法。请结合下面的历史时间轴示意图和所学知识回答问题。

时间轴一：

时间轴二：

请回答：

聚落形成与发展受到自然条件影响尤为突出，聚落发展的有利因素有哪些？

设l，m，n表示三条直线，α，β，γ表示三个平面，给出下列四个命题：
①若l⊥α，m⊥α，则l∥m；
②若m⊂ β，n是l在β内的射影，m⊥l，则m⊥n；
③若m⊂α，m∥n，则n∥α；
④若α⊥γ，β⊥γ，则α∥β.
其中真命题为