2×（－$\frac{1}{2}$）的结果是（     ）

 下列有关实验的叙述错误的是（　　）

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阅读文言文，完成后面题目。

       秦王谓唐雎曰：“寡人以五百里之地易安陵，安陵君不听寡人，何也？且秦灭韩亡魏，而君以五十里之地存者，以君为长者，故不错意也。今吾以十倍之地，请广于君，而君逆寡人者，轻寡人与？”唐雎对曰：“否，非若是也。安陵君受地于先王而守之，虽千里不敢易也，岂直五百里哉？”

       秦王怫然怒，谓唐雎曰：“公亦尝闻天子之怒乎？”唐雎对曰：“臣未尝闻也。”秦王曰：“天子之怒，伏尸百万，流血千里。”唐雎曰：“大王尝闻布衣之怒乎？”秦王曰：“布衣之怒，亦免冠徒跣，以头抢地耳。”唐雎曰：“此庸夫之怒也，非士之怒也。夫专诸之刺王僚也，彗星袭月；聂政之刺韩傀也，白虹贯日；要离之刺庆忌也，仓鹰击于殿上。此三子者，皆布衣之士也，怀怒未发，休祲降于天，与臣而将四矣。若士必怒，伏尸二人，流血五步，天下缟素，今日是也。”挺剑而起。

（选自《唐雎不辱使命》）

    Hello, everyone! Tomorrow we'll go for a trip in the mountains. Before we start, there are some ____ and suggestions for you. First, everyone should ____ sports shoes. Second, when you walk in the mountains, you mustn't walk too ____ "to the edge of the path. Third, you must keep ____ or you'll get lost. Fourth, bring some water with you, or you'll be ____.

    We need to put up a tent to stay at night in the mountains. Be careful of the ____ when you are in the forest. Although some animals look ____ they are really dangerous. If you meet animals, don't move suddenly. Just keep still until they ____. Remember to hang your food in the tree before you go to sleep Hope everyone can have a good trip.