老师,遮罩只能弄成白色么?怎么才能渐变呢?

<svg viewBox="0 0 400 200" xmlns="http://www.w3.org/2000/svg">

    <defs>

        <linearGradient id="Gradient" >

            <stop offset="0" stop-color="red" stop-opacity="0" />

            <stop offset="1" stop-color="yellow"  stop-opacity="1" />

        </linearGradient>

<mask id="mask">

<rect x="0" y="0" width="200" height="100" fill="url(#Gradient)" />

</mask>

    </defs>

    <circle cx="100" cy="100" r="100"  mask="url(#mask)" />

</svg>