关于JS的this和构造函数问题

Question

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);            // 为什么会是"hello"?    alert("stu-hello");}var s = new Student();s.say();如代码中注释，superSay.call(this)为什么会是People.prototype.say函数被调用？this指向谁？