Consider the competition among private insurance companies that offer Medicare Part D insurance plans to senior citizens in the U.S. Which of the following would be expected to DECREASEthe aggressiveness of price rivalry in the Medicare Part D insurance market?

1. a) Under the law, insurers have little flexibility in terms of what drugs are covered by their insurance plans.
2. b) Each of the insurers is required to provide information on their plans to Medicare in a standardized format; Medicare publishes this standardized information online and in publications mailed to every Medicare-eligible citizen.
3. c) Over the last two decades, there has been significant entry by new insurers into the prescription drug insurance industry.
4. d) Some insurers have obtained exclusive endorsements from third parties such as the American Association of Retired Persons (AARP, the largest advocacy group in the U.S. representing senior citizens) and Prevention magazine (a well-known magazine focusing on health issues relevant to seniors).