18. In the diagram, $P Q R S$ is a square of side $10 \mathrm{~cm} . T$ is a point inside the square so that $\angle S P T=75^{\circ}$ and $\angle T S P=30^{\circ}$. What is the length of $T R$ ?
A $8 \mathrm{~cm}$
B $8.5 \mathrm{~cm}$
C $9 \mathrm{~cm}$
D $9.5 \mathrm{~cm}$
E $10 \mathrm{~cm}$