18. The sum of the areas of the squares on the sides of a right-angled isosceles triangle is $72 \mathrm{~cm}^{2}$. What is the area of the triangle?
A $6 \mathrm{~cm}^{2}$
B $8 \mathrm{~cm}^{2}$
C $9 \mathrm{~cm}^{2}$
D $12 \mathrm{~cm}^{2}$
E $18 \mathrm{~cm}^{2}$