2007 AMC 10B Problem 9

A cryptographic code is designed as follows. The first time a letter appears in a given message it is replaced by the letter that is 1 place to its right in the alphabet (assuming that the letter A is one place to the right of the letter Z). The second time this same letter appears in the given message, it is replaced by the letter that is 1+2 places to the right, the third time it is replaced by the letter that is 1+2+3 places to the right, and so on. For example, with this code the word "banana" becomes "cbodqg". What letter will replace the last letter s in the message \text{"Lee's sis is a Mississippi miss, Chriss!"?}

\textbf{(A) } g \qquad\textbf{(B) } h \qquad\textbf{(C) } o \qquad\textbf{(D) } s \qquad\textbf{(E) } t

Full credit goes to MAA for authoring these problems. These problems were taken on the AOPS website.