In order to estimate the value of $x-y$ where $x$ and $y$ are real numbers with $x\textgreater y\textgreater0$, Xiaoli rounded $x$ up by a small amount, rounded $y$ down by the same amount, and then subtracted her rounded values. Which of the following statements is necessarily correct? (2012 AMC 10B Problem, Question \#6) 
选项为:A:Her estimate is larger than $x-y$  B:Her estimate is smaller than $x-y$  C:Her estimate equals $x-y$  D:Her estimate equals $y-x$  E:Her estimate is $0$  请回答其中一个选项.