HELP ME IF I FAIL THIS I WONT GET MY SURPRISE.

The function p(d)=5·4^(d−1) represents the number of people who receive a certain e-mail on day d of an e-mail experiment. What does the value 5 represent in this situation?

Every day, the number of people receiving the e-mail increases by 5.

Every day, the number of people receiving the e-mail increases by 5%.

There are 5 people running the experiment.

Five people received the e-mail on the first day of the experiment.