In the figure shown, P Q R S has three sides of equal length and S R=16 \mathrm{~cm}. If the perimeter of P Q R S is 40 \mathrm{~cm}, then the length of P Q is
Answer Choices
A. 6 \mathrm{~cm}
B. 7 \mathrm{~cm}
C. 8 \mathrm{~cm}
D. 9 \mathrm{~cm}
E. 10 \mathrm{~cm}