Problem
Mock AMC 8 #1 - Problem 21
Let k be the largest positive integer such that 7^{4k} is a divisor of (777!)^2. What is the sum of the digits of k?
(\textbf{A})\;9\quad(\textbf{B})\;12\quad(\textbf{C})\;11\quad(\textbf{D})\;10\quad(\textbf{E})\;13
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