The adage, "there is no such thing as a free lunch," is used to illustrate the principle that select one: a. goods are scarce. b. people face tradeoffs. c. income must be earned. d. households face many decisions.

c. income must be earned.

Explanation:

This means one is expected to go to work everyday as stated in their contract and when they get there they need to do their work.

One can not go to work and just sit at the office and discuss current affairs with the colleagues unless it is lunch time.

When one works they get paid ,however when they stay at home and not work or go to work they can not expect to be paid .

the following key ideas correctly express the key sentence.

restate key idea: most people can learn to swim in ten short lessons.

restate key idea: the majority of water-accident victims require mouth-to-mouth breathing.

restate key idea: an empty, tightly closed gallon jug will support a tired swimmer.  

restate key idea: first aid is to prevent accidents as well as assist in rescue.

explanation: the key idea needs to have the same topic as the key sentence (drowning and safety).

i think its d, but grammarly says none

explanation: