在使用NSJSONSerialization时从NSDictionary获得null。

I am new to developing and stackoverflow please help me. i am trying to do a simple application where the YQL link is used to get local data and display it in table format. For that i am converting the data into dictionary , later i want to send it into table. But when i tried to convert data to Dictionary it says null. Please help me. Check the Screenshot below. Thanks in advance.I am new to developing and stackoverflow please