function People() {}People.prototype.say = function () {    alert("hello");}function Student() {}Student.prototype = new People();var superSay = Student.prototype.say;Student.prototype.say = function () {    superSay.call(this);            // 為什么會是"hello"?    alert("stu-hello");}var s = new Student();s.say();如代碼中注釋，superSay.call(this)為什么會是People.prototype.say函數被調用？this指向誰？