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背景材料： 下面是铝热反应的改进实验。 （1）实验用品：2g氧化铁和铝粉的混合物（质量比5：2）、氯酸钾、无水乙醇、粉笔1支、烧杯、镊子、药匙、玻璃片、带铁夹的铁架台、盛细沙的蒸发皿、磁铁、火柴、小刀。 （2）实验步骤 ①用小刀在干粉笔上挖出宽约0.5cm、深0.3cm的浅槽后，将粉笔浸泡有无水乙醇的烧杯中。 ②用镊子取出浸透乙醇的粉笔于玻璃片上，用药匙把2g混合好的铝热剂平铺在粉笔槽中，上面再铺少许氯酸钾。 ③将盛药品的粉笔倾斜固定在铁夹上，并在下方放置盛细沙的蒸发皿。 ④用火柴点燃粉笔，观察实验现象。 问题： （1）该实验可以观察到的现象有哪些？写出发生反应的化学方程式。 （2）该改进实验在教学中有哪些优点？
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【单选题】The Mayan calendar far more accurate than the European calendars of the time most probably because ______.
A.
the Mayan numbers enabled the Mayan priests to study planets
B.
the European countries had no interest in founding a calendar
C.
the European calendars of the time probably were made by a guess work
D.
the year was divided into 18 months of 20 days each with five days left over
【单选题】This article is mainly about ______.
A.
the Mayan calendar and the ancient Mayan buildings
B.
the Mayan Indians and the Mayan calendar
C.
the intelligent Maya and their many achievements
D.
the ancient Mayan cities and temples
【单选题】What can be inferred about the Mayan culture?
A.
Mayan civilization began to decline around 900 AD.
B.
Historians are not sure about the reason for the decline.
C.
The study of Mayan civilization has become very popular now.
D.
Mayan civilization will flourish again in the future.
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