Given: $a-b\textgreater a$ and $a+b\textless b$. Which of the following must be true?~\uline{~~~~~~~~~~}~  $I$. $a b$ is negative  $II$. $a+b$ is negative  $III$. $a-b$ is negative 
选项为:A:$I$ only  B:$I$ and $III$ only  C:$II$ only  D:$I$, $II$, $III$  请回答其中一个选项.