Which of the following are examples of tit-for-tat strategies?

a. A friend forgets to send you a birthday card this year, so you decide not to send one to her on her next birthday.

b. Your friend let you borrow her class notes last week, so you decide to skip class again this week.

c. You and your roommate take turns buying toilet paper, milk, and other shared items.

d. Your book club chooses a book you don’t want to read, so you decide not to go this month.