Susan is a competitive soccer player. During a recent game she heard something snap in her leg and now she is in terrible pain and is having trouble walking. She has discovered that her health insurance will not allow her to seek treatment from an orthopedic specialist until she has first seen her family doctor and received a referral. This is an example of what form of medical cost control technique used by some managed care plans?

a$)$ A preventative care emphasis

b$)$ A gatekeeper function

c$)$ Reasonable and customary fees

d$)$ A modified fee$-$for$-$service