In the $5$W$1$H event management framework, $$WHY$$ is often the most important question event managers have to answer. For example, the objective of a blood donation event is to encourage more people to give blood. Which of the following is the best measurement of success of this event?

a$.$

All of the above

b$.$

Number of social media engagement on the celebrities who attended the event.

c$.$

Percentage of event attendees from last year who come back to attend this year$$s event.

d$.$

Increase in the number of new blood donors.