Researchers Studying Expertise Sometimes Focus on the Abilities of Professionals

Researchers studying expertise sometimes focus on the abilities of professionals (athletes,chess masters,etc. ) because:

A) the researchers have a great deal of control over the environmental variables affecting such people.
B) thee performance of professionals is usually easy to quantify.
C) these areas only require cognitive skills,which are the most interesting to researchers.
D) professionals all have roughly the same level of expertise,which eliminates variability in the data.