阅读下列短文，从每题所给的四个选项（A、B、C和D）中选出最佳选项。

    Four seasons make a year. They a re spring, summer, autumn and winter. 

    Spring is a beautiful season. It's from March to May. In spring, trees turn green and flowers come out. Everything looks new.

    Summer is a happy season. It's from June to August. The weather is hot, so people usually wear cool clothes, go swimming and eat ice cream.

    Autumn is a colourful season. It is from September to November. In autumn, leaves on trees turn yellow. Sometimes they are yellow and red. The colours are beautiful.

    Winter is the last season. It's from December to February. In China, the weather is not the same in winter. In Beijing, it's very cold. It often snows. People often play with snow and go skating in rivers and lakes. Skating is their favorite sport in winter. But in Haihan, it's not very cold. It's very warm. People there can go swimming in the sea.

A、y=2^x﹣1；  B、y=2^x﹣1； C、y=﹣2^x；D、y=﹣2^﹣x；E、y=（ $\frac{1}{2}$ ）^x；F、y=（ $\frac{1}{2}$ ）^﹣x

某商场试销一种商品，成本为每件200元，规定试销期间销售单价不低于成本单价，且获利不得高于50%，一段时间后，发现销售量y（件）与销售单价x（元）之间的函数关系如下表：