【考查知识点：复合函数】 设函数，则f（x）=（     ）．

<span style、"font-family: 'times new roman', times, serif;"><em>x²</em>-3</span>

<span style、"font-family: 'times new roman', times, serif;"><em>x²</em>-4</span>

<span style、"font-family: 'times new roman', times, serif;"><em>x²</em>-5</span>

<span style、"font-family: 'times new roman', times, serif;"><em>x²</em>-6</span>