21. There are three squares: large, medium, and small, with side lengths all being integer centimeters. The perimeter of the small square is less than the side length of the medium square. When these two squares are placed on the large square (as shown in the figure), the area of the part of the large square that is exposed is 10 square centimeters (the shaded part in the figure). Therefore, the area of the large square is ( ) square centimeters.
A. 25
B. 36
C. 49
D. 64
【Answer】Solution: According to the analysis, the area of one shaded part is $10 \div 2=5$ square centimeters. Since they are all integers, it can only be $1 \times 5$. Therefore, the area of the large square $=(1+5) \times(1+5)=6 \times 6=36$ square centimeters. Hence, the answer is: $B$.