阅读短文，掌握大意，然后从A、B、C、D四个选项中，选出可以填入空白处的最佳选项。

    A little sports shop is near my house. It sells (卖) many good things for  1 . At the shop, we can buy soccer balls,  2 , basketballs and so on. Today my  3 and I go to the4  to buy a tennis racket. There are a lot of 5 . And they have different  6   . I think every racket  7  very nice. My sister chooses (选择) a black one, because black is her favorite8  . You know, my sister is a big sports fan. She has a great sports    9   at home-five soccer balls, four basketballs and many tennis balls. She really likes   10  sports.

某校九年级两个班各为玉树地震灾区捐款1 800元，已知2班比1班人均捐款多4元，2班的人数比1班的人数少10%，求两个班人均捐款各多少元？

设a,b,c为正实数,且满足abc=1,试证明:$\frac{1}{a^3(b+c)}$  +$\frac{1}{b^3(a+c)}$  +$\frac{1}{c^3(a+b)}$  ≥$\frac{3}{2}$ .

$\frac{1}{2}+\frac{1}{5}$ =____

$\frac{1}{3}+\frac{1}{4}$ =____