9. A square has side-length $10 \mathrm{~m}$. It is divided into parts by three straight line segments, as shown. The areas of the two shaded triangles are $P \mathrm{~m}^{2}$ and $Q \mathrm{~m}^{2}$.
What is the value of $P-Q$ ?
A 0
B 1
C 2
D 5
E 10