Why Is the Market for Cardiologists (Heart Specialists) More Likely

Why is the market for cardiologists (heart specialists) more likely to be oligopolistic than the market for family practice doctors?

A) There are fewer potential patients for cardiologists than for family practice doctors.
B) Higher incomes for family practice doctors create greater incentive for medical students to enter the field of family practice.
C) The extra years of training needed to become a cardiologist create a higher barrier to entry than to the field of family practice.
D) Cardiologists have lower ATC and MC curves than family practice doctors.