Which of the following meet the economic definition of an informative signal?
a. A man asks for woman's hand in marriage and gives her a large diamond ring.
b. The same man takes his fiancée for a walk along the beach where he promises he will stop driving fast cars.
c. The same woman promises her employer that she will finish an important report before the wedding date.
d. The woman offers to accept a lower salary in exchange for a large bonus if she meets the completion deadline.