If $n$ and $m$ are integers and $n^{2}+m^{2}$ is even, which of the following is impossible? (2014 AMC 8 Problem, Question \#13) 
选项为:A:$n$ and $m$ are even  B:$n$ and $m$ are odd  C:$n+m$ is even  D:$n+m$ is odd  E:none of these are impossible  请回答其中一个选项.