Read the excerpt from Dr. Martin Luther King's "Letter from Birmingham Jail."

"If I sought to answer all the criticisms that cross my desk, my secretaries would have little time for anything other than such correspondence in the course of the day, and I would have no time for constructive work. But since I feel that you are men of genuine good will and that your criticisms are sincerely set forth, I want to try to answer your statement in what I hope will be patient and reasonable terms."

What is the author's purpose for including these sentences?

A. To describe the productivity of his daily routine
B. To emphasize the significance of his critics' letter
C. To establish the impressive credentials of his staff
D. To explain the nature of his profession