Factor the expression 21x^{2} – 36x + 15 by first factoring out a monomial. Which of these is the correct solution?

(A) 3(7x – 5)(x – 3)

(B) 21(7x – 5)(x – 1)

(C) 3(7x – 5)(x – 1)

(D) This expression cannot be factored.