阅读理解

    Turn left at the second crossing if you drive along Center Street north to south. If you come by bus, you can get off(下车) at the B. Street Bus Station. You can't miss it!

    Please come to the black door. Our party will be on the farm behind the house. Put your gifts into the small tent on the grass(草地). Remember : This is a surprise party, so do not let him know anything about it by the party.

    We will have a buffet lunch(自助餐) and later all kinds of interesting games for everybody!

    Music Show at 4 P.M.

    Bring your guitars, violins, drums and anything that makes sound！

    Please let me know by July 4^th if you can make it.

有关中南半岛的叙述，正确的是（     ）

已知函数 $f(x)=\{\begin{array}{l}{3}^{-x}(x\le 0)\\ \sqrt{x}(x>0)\end{array}$ ，若函数 $g(x)=f(x)-\frac{1}{2}x-b$ 有且仅有两个零点，则实数 $b$ 的取值范围是____.