Which of the following is true of Social Security (check all that apply)

a. Participational is optional: you can choose to not pay social security taxes and not receive social security benefits

b. A worker's spouse and unmarried children younger than age 18 are eligible for retirement benefits in the event of the worker's death

c. Social Security's share of the federal budget has been decreasing over time

d. Survivor benefits can be paid to dependents of a deceased worker who has not yet retired