燃煤发电厂利用反应2SO₂+2CaCO₃+X=2CaSO₄+2CO₂来减少SO₂的排放，其中X的化学式为（）

   Football is, I do think, the most favorite game in England. It is one of the important  1 to see. Rich and poor, young and old, one can see them. To a stranger, one of the most surprising things about football in England is the great  2  of the game which even the smallest boy seems to have. He can  3you the names of the players in most of the important teams; he has pictures of them and knows the  4  of Large numbers of matches. He will tell you who he hopes will  5  such a match, and his ideas about football are usually as good as those of men three or four times his age.

实验是检验化学原理的“最高法庭”．下列化学实验基本操作中正确的是（　　）

以直角三角形的三边为边长分别向外作正方形，已知其中两个正方形的面积分别为20和16，则第三个正方形的边长为（       ）