What does the passage mainly discuss?

Dear all,

The following text is quoted from my university entrance mocking test. Could you take a look at it and help me to find the best answer to the question following? Thank you very much.

We get great pleasure from reading. The more advanced a man is, the greater delight he will find in reading. The ordinary man may think that subjects like philosophy or science are very difficult and that if philosophers and scientists read these subjects, it is not for pleasure but for some other ends. But this is not true. The mathematician finds the same pleasure in his mathematics as the school boy in an adventure story. For both it is a play of the intellect or the imagination, a mental recreation and exercise.

The pleasure that is derived from an exercise to the mental faculties is common to all kinds of reading. But different types of books give us different types of pleasure. First in order of popularity is novel-reading. Novels contain pictures of imaginary men and women in imaginary situations, and they give us an opportunity of escaping into a new world which is very much like our world and yet different from it. Here we seem to live a new life, and the experience of this new life gives us a thrill of pleasure.

Next in order of popularity are travel books, biographies and memoirs. These tell us tales of places we have not seen and of great men in whom we are interested. Some of these books are as wonderful as novels, and they have an added value that they are true. Few novels can be more thrilling than the lives of Napoleon or Abraham Lincoln, and we shall devour such lives as we devour novels. Such books give us knowledge, but we also find immense pleasure in knowing details of lands we have not seen and of great men whom we have only heard of.

Reading is one of the greatest enjoyments of life. To the book-lover nothing is more fascinating than a favorite book, and the ordinary educated man who is interested and absorbed in his daily occupation wants occasionally to escape out of his drudgery into the wonderland of books for recreation and refreshment.
(Source: http://importantindia.com/)

What does the passage mainly discuss?
A. Different kinds of books
B. Different kinds of reading
C. Reading as an exercise for the brain
D. Reading as a pleasurable activity.
My choice is D but the key given is A. As a native English speaker, could you tell me the best answer? I'll appreciate your help.