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
Cause of B/L recurrent laryngeal nerve palsy are -a) Thyroid Cab) Thyroid surgery c) Bronchogenic cancer d) Aortic aneurysm 
Select and explain. 
A. ac, B. a, C. ab, D. bc
C: ab
Answer: C: ab 
Explanation: Causes of bilateral recurrent laryngeal nerve paralysis are Thyroid surgery, Carcinoma thyroid, cancer cervical esophagus, cervical lymphadenopathy.