For each of following scenarios, identify whether it is an example of a demand-side, supply-side, regulatory, or deterrence strategy to limit competition in the market. 

a. Delta Airlines provides frequent flyer miles to members when they travel. These miles can be redeemed for travel and upgrades on Delta flights. 

b. Toyota, Ford, and General Motors have invested in large manufacturing plants that help them efficiently produce millions of cars each year. 

c. The top-selling pharmaceutical drug in the world in 2017 was Humira, which is used to treat rheumatoid arthritis. Abbvie has a patent for Humira. 

d. Between 2015 and 2018, Facebook had increased the number of active monthly users from 1.4 billion to 2.19 billion.