12. In the diagram, $P Q R S$ is a square, $P S T$ is an equilateral triangle and $S R U V W$ is a regular pentagon.
What is the size of angle WTS?
A $35^{\circ}$
B $36^{\circ}$
C $37^{\circ}$
D $38^{\circ}$
E $39^{\circ}$