A.200 B.300 C.500 D.404
A.0 B.1 C.2 D.3 E.4
A.xmlHttpRequest. callBackMethod=onreadystatechange; B.xmlHttpRequest. onreadystatechange(callBackMethod); C.xmlHttpRequest. onreadystatechange(new function(){callBackMethod }); D.xmlHttpRequest. onreadystatechange= callBackMethod;