This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: ENT
Which of the following muscle is known as safety muscle of larynx? 
Select and explain. 
A. Cricothyroid, B. Thyroarytenoid, C. Lateral cricoarytenoid, D. Posterior cricoarytenoid
D: Posterior cricoarytenoid
Answer: D: Posterior cricoarytenoid 
Explanation: Posterior cricoarytenoid is known as safety muscle of larynx. It is so called because being the only abductor of the vocal cords it keeps the rima glottis open to allow air to enter the lungs. Adductors of vocal cord are: Cricothyroid Thyroarytenoids Lateral cricoarytenoid Transverse arytenoids Relaxant of vocal cords: Vocalis Thyroarytenoid