In What Way(s)was Baghdad the Cultural and Learning Center of the Muslim

In what way(s) was Baghdad the cultural and learning center of the Muslim Empire?

A) Collections and translations from earlier works were collected.
B) Major mathematical and scientific discoveries resulted from research.
C) An important base of knowledge was provided for the future of Europe.
D) Because of the use of paper, books became available to the public on many subjects.
E) All of these.