Let $a, b, c, d$, and $e$ be five consecutive terms in an arithmetic sequence, and suppose that $a+b+c+d+e=30$. Which of $a, b, c, d,$ or $e$ can be found?
$\textrm{(A)} \ a\qquad \textrm{(B)}\ b\qquad \textrm{(C)}\ c\qquad \textrm{(D)}\ d\qquad \textrm{(E)}\ e$
Let $f$ be the common difference between the terms.

$a=c-2f$
$b=c-f$
$c=c$
$d=c+f$
$e=c+2f$
$a+b+c+d+e=5c=30$, so $c=6$. But we can't find any more variables, because we don't know what $f$ is. So the answer is $\textrm{C}$.