What is the completely factored form of x2 – 16xy + 64y2?

What is the completely factored form of x2 – 16xy + 64y2?
A. xy(x – 16 + 64y)
B. xy(x + 16 + 64y)
C. (x – 8y)(x – 8y)
D. (x + 8y)(x + 8y)