单项选择题
document.onmouseup = function(){ this.onmousemove = null; this.onmouseup = null; } ,这里的this指向()
A.window对象 B.document对象 C.function对象 D.以上说法都正确
A.Object B.Error C.Function D.loading
A.3,4 B.4,4 C.4,5 D.undefined,undefined