21. Mara has a large white cube with a side length of 10 feet, and she has enough green paint to cover 300 square feet. Mara wants to paint a white square in the center of each face and use all the paint around the squares. Then the area of each white square is ( ) square feet.
(A) $5 \sqrt{2}$
(B) 10
(C) $10 \sqrt{2}$
(D) 50
(E) $50 \sqrt{2}$
21. D.
$$
\frac{10 \times 10 \times 6-300}{6}=\frac{300}{6}=50 \text {. }
$$