To simplify the expression \((53x + 55y) + (-17x + 45y)\), we can combine like terms.
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Combine the \(x\) terms: \[ 53x + (-17x) = 53x - 17x = 36x \]
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Combine the \(y\) terms: \[ 55y + 45y = 100y \]
Putting it all together, we have: \[ 36x + 100y \]
Thus, the simplified expression is \(\textbf{C. } 36x + 100y\).