<3 English timed test

Read this sentence from Neil deGrasse Tyson's preface to his book The Pluto Files:

Gathered here in one place is a record of Pluto's rise and fall from planethood, given by way of media accounts, public forums, cartoons, and letters I received from disgruntled schoolchildren, their teachers, strongly opinionated adults, and colleagues.
Why would this be considered an example of expository writing?
A. It argues that Pluto should not be considered a planet.
B. It fictionalizes the story of Pluto's history.
C. It gives an opinion of Tyson's book about Pluto.
D. It informs the reader about Pluto's planetary status.