What is the main difference between calling the commands `ansible' and `ansible-playbook' on the command line?

A. `ansible' is for setting configuration and environment variables which `ansible-playbook' will use when running plays.
B. `ansible-playbook' is for running entire Playbooks while `ansible' is for calling ad-hoc commands.
C. `ansible-playbook' runs the playbooks by using the `ansible' command to run the individual plays
D. `ansible' is for running individual plays and `ansible-playbook' is for running the entire playbook.