In the diagram, \triangle P Q R has \angle P Q R=120^{\circ}. Also, \angle Q P S=\angle R P S and \angle Q R S=\angle P R S. (In other words, S P and S R bisect \angle Q P R and \angle Q R P, respectively.) What is the measure of \angle P S R ?
Answer Choices
A. 130^{\circ}
B. 120^{\circ}
C. 140^{\circ}
D. 160^{\circ}
E. 150^{\circ}