完形填空。 The amount of usable water has alw
题型：完形填空
问题：
| 完形填空。 | ||||
| The amount of usable water has always been of great interest in the world. 1 springs and streams sometimes means control, particularly in the 2 areas like the desert. The control is possible even without possession of large areas of 3 land. In the early days of the American West, gun fights were not 4 for the water resources (资源). And laws had to be 5 to protect the water rights of the 6 and the use of the water resources accordingly. 7 is known to us all, there is not 8 water in all places for everyone to use as much as he likes. Deciding on the 9 of water that will be used in any particular period 10 careful planning, so that people can manage and use water more 11 . Farmers have to change their use of or demand for water 12 the water supply forecast (预报). The 13 water supply forecast is based more on the water from the 14 than from the below. Interest is 15 in the ways to increase rainfall by man-made methods, and to get water from the winter snow on mountain 16 . With special equipment, some scientists are studying the ways in which the mountain snow can be 17 , and with the help of a repeater station, they send the 18 data (数据) to the base station. The operator at the base station can get the data at any time by 19 a button. In the near future, the forecast and use of water 20 probably depend on the advance knowledge of snow on mountains, not of water underground. | ||||
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时，闭合回路的感应电动势多大？此时作用于金属杆的外力F多大？