How Has the Link Between Exercise and Cognitive Functioning Been

How has the link between exercise and cognitive functioning been examined?

A) Researchers have been limited to correlational studies.
B) Researchers have been able to use experimental studies but these have been limited to animal subjects and subsequent performance.
C) Researchers have been able to examine brain activation levels and exercise only when conducting animal studies.
D) Researchers have been able to examine brain activation levels and exercise in experimental studies with human participants.