A New Type of Pain Reliever Is Administered to 30

Multiple Choice

A new type of pain reliever is administered to 30 consenting post-operative patients in various hospitals.Although the pain reliever has already been tested for safety and effectiveness,this experiment is to observe and categorize any side-effects.Because of maturity and body mass,it is decided to test the adults separately from the children.
The grouping of the adults separate from the children is an example of what?

A) Stratifying
B) Controlling
C) Matching
D) Reduction of confounding factors
E) Blocking