AHSME 1970 Problem 25

For every real number $x$, let $[x]$ be the greatest integer which is less than or equal to $x$. If the postal rate for first class mail is six cents for every ounce or portion thereof, then the cost in cents of first-class postage on a letter weighing $W$ ounces is always $\text{(A) } 6W \text{(B) } 6[W] \text{(C) } 6([W]-1) \text{(D) } 6([W]+1) \text{(E) } -6[-W]$