以下有关牙合支托的描述哪一项是不正确的？（） A.支托宽度是前磨牙颊舌径的1/2

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A．It is known that the“Great Water”has also been a frontier river（边境河）. B．Probably the river remembers many happy things，and also many sad things. C．The Mississippi is the greatest river in the United States. D．The Mississippi has always had an important part in American history． E．Great rivers are likely to suffer floods. F．As we know．it's the third longest river in the world．

 
The Mississippi
The Mississippi is a romantic river whose relationship with man goes back beyond its discovery by the Spaniards in the 16th century．Indians used the river as a highway and as a source of food．and it was they who gave it its name“misi-sipi”，meaning“river of many fishes”．European explorers called it the Mississippi．Today, it is often called“Old Man River”．
  45 ．Flowing from north to south，it gathers many waters on its way．You will easily understand why it is called“Old Man River”，or the“Father of Waters”．A lot of early setters built their homes on the land along its waters．The Mississippi was and is the old man of all American people，too
  46 ．In 1927 the Mississippi flooded 26 thousand acres，sweeping away farms，towns，everything in its path．In 1938 its floods drowned or killed 200 people and made millions homeless．Today the river has largely been controlled．
  47 ．Pioneers who first reached its banks wondered not only where it went，but what lay beyond．In 1764 the French founded a city on the right bank of the river, and named it after their king，Louis XV．This city, named St Louis，became the jumping-off place for the adventurous men and women who opened up the Great Plains，and the way to the Far West．
  48 ．One of the saddest memories may be that of the blacks who were carried down the river and sold as slaves．The river also remembers that their sobbing and crying voices once passed over its waters．
  49 ．Today, the river is still an important part of the American economy．Goods are carried up and down the river to get to other parts of the country and the world．