As the main perpetrator of the offence, principal is usually the person who commits the actus reus. Where more than one person is directly responsible for the actus reus, there may be more than one principal; those are known as joint principals. In general, criminal law systems distinguish three forms of principals: principal, joint principal and indirect principal. To put it another way, whoever commits an offence individually or through another person is liable as a principal. The correct answer is B.