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: Social & Preventive Medicine
Which of the following is/are non-random sampling methods-a) Quota samplingb) Stratified random samplingc) Convenience Samplingd) Cluster Sampling 
Select and explain. 
A. a, B. bc, C. ad, D. ac
D: ac
Answer: D: ac 
Explanation: None