Which of these eased restrictions on preferred provider organizations $($PPOs$)$ and allowed subscribers to seek health care from providers outside the PPO $?$

a$.$ Health Maintenance Organization Assistance Act of $1973$

b$.$ Preferred Provider Health Care Act of $1985$

c$.$ The Tax Equity and Fiscal Responsibility Act of $1983$

d$.$ Amendment to the HMO Act of $1973$