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
Nasopharyngeal carcinoma causes deafness by -a) Blocking the Eustachian tubeb) Serous otitis mediac) Temporal bone metastasisd) Radiation 
Select and explain. 
A. ab, B. a, C. ad, D. bc
A: ab
Answer: A: ab 
Explanation: Nasopharyngeal carcinoma can cause obstruction of eustachian tube which results in serous otitis media and conductive deafness.