If your [average](https://artofproblemsolving.com/wiki/index.php/Average) score on your first six mathematics tests was $84$ and your average score on your first seven mathematics tests was $85$, then your score on the seventh test was
$\text{(A)}\ 86 \qquad \text{(B)}\ 88 \qquad \text{(C)}\ 90 \qquad \text{(D)}\ 91 \qquad \text{(E)}\ 92$
If the average score of the first six is $84$, then the [sum](https://artofproblemsolving.com/wiki/index.php/Sum) of those six scores is $6\times 84=504$. 
The average score of the first seven is $85$, so the sum of the seven is $7\times 85=595$
Taking the difference leaves us with just the seventh score, which is $595-504=91$, so the answer is $\boxed{\text{D}}$