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: Pharmacology
Which of the following mucolytic contains free sulphydryl group 
Select and explain. 
A. Guanfacine, B. Ambroxal, C. Carbocistene, D. N-Acetyl cysteine
D: N-Acetyl cysteine
Answer: D: N-Acetyl cysteine 
Explanation: None