Apex Corporation Controls the Sole Source of a Critical Raw

Apex Corporation controls the sole source of a critical raw material required to produce ACE inhibitors, one of the types of drugs used to control blood pressure. The other major class of blood pressure medications, called beta-blockers, do not require the material supplied by Apex. In its industry value chain, which of the five forces places a limit on Apex's competitive advantage?

A) New entrants into the market as producers of ACE inhibitors.
B) Raw material substitutes.
C) Customers/patients able to use beta-blockers to control blood pressure.
D) Other raw material suppliers.