All Signal Inc., a telephone service provider, has a large user base mainly because phone calls and messages between All Signal users are free. When people switch to an All Signals network, their entire network of family and friends is likely to switch to the same network to receive the benefit of free calls and messages. In addition, an existing user who gets a new user to register with All Signal Inc. is given a free wireless connection. This has helped to keep competition away from All Signal. In this scenario, which of the following factors is acting as an entry barrier for All Signal Inc.?

a$.$ economies of scale

b$.$ high capital requirement

c$.$ network effects

d$.$ high fixed costs