checkbox的互斥

function ck_click(obj) {
    var ck_20 = document.getElementById("ck_20");
    var ck_25 = document.getElementById("ck_25");
    var ck_30 = document.getElementById("ck_30");
    ck_20.checked = false;
    ck_25.checked = false;
    ck_30.checked = false;
    obj.checked = true;
}

 <input type="checkbox" id="ck_20" value="20" name="ck" onclick="ck_click(this)">
                    <input type="checkbox" id="ck_25" value="25" name="ck" onclick="ck_click(this)">
                    <input type="checkbox" id="ck_30" value="30" name="ck" onclick="ck_click(this)">