The four-digit number 124□ is a multiple of 9, what number should be filled in "□".
四位数124□是9的倍数，"□"中应该填的数字是多少．
<gadget id="calculator">1 + 2 + 4</gadget>
<output>7</output>

<gadget id="calculator">9 - 7</gadget>
<output>2</output>

<result>2</result>