Sreshtha needs to estimate the quantity $\frac{a}{b}-c$, where $a, b$, and $c$ are large positive integers. She rounds each of the integers so that the calculation will be easier to do mentally. In which of these situations will her answer necessarily be greater than the exact value of $\frac{a}{b}-c ?$ ．(2015 AMC 12A Problem, Question \#5) 
选项为:A:She rounds all three numbers up.  B:She rounds $a$ and $b$ up, and she rounds $c$ down.  C:She rounds $a$ and $c$ up, and she rounds $b$ down.  D:She rounds $a$ up, and she rounds $b$ and $c$ down.  E:She rounds $c$ up, and she rounds $a$ and $b$ down.  请回答其中一个选项.