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Which of the following shows the correct first step to solve x^2 - 16x = -21 by completing the square?

A)x^2 - 16x + 64 = -21

B) x^2 - 16x + 64 = -21 + 64

C) x^2 - 16x + 16 = -21 + 16

D) x^2 - 16x + 8 = -21 + 8