Importance of span of control

Which (if any) of the following scenarios is the result of a natural monopoly? instructions: you may select more than one answer. a. patent holders of genetically modified seeds are permitted to sue farmers who save seeds from one planting season to the next. b. doctors in the united states are prohibited from practicing without a medical license. c. there is one train operator with service from baltimore to philadelphia. d. coal is used as the primary energy in a country with abundant coal deposits.