A pharmaceutical company, interested in measuring how often physicians prescribe a certain drug, has selected a simple random sample from each of two groups: M.D. (medical doctors) and D.O. (doctors of Osteopathic). What type of sampling is this?
A. Simple random sampling
B. Stratified random sampling
C. Cluster sampling
D. None of the above

Which of the following sampling plans uses random methods of selections?
A. Cluster sampling
B. Simple random sampling
C. Stratified random sampling
D. All of the above