CAYLEY 2021 Problem 10

In the diagram, pentagon T P S R Q is constructed from equilateral \triangle P T Q and square P Q R S. The measure of \angle S T R is equal to
caley 2021 10

Answer Choices
A. 10^{\circ}
B. 15^{\circ}
C. 20^{\circ}
D. 30^{\circ}
E. 45^{\circ}