They are Anne and Margot's homeschooling assignments.
They are maps of the German forces.
They are letters from Anne's friends at school.
They are pictures of Anne's favorite celebrities.
Because Anne Frank appears very young and international audiences are very sympathetic to children.
Because Anne's smiling face mirrors the optimistic message of the book that the publishers wanted to focus on.
Because Anne's very normal appearance forced international readers to recognize that the Jews being killed were regular people.
All of the above.
Anne and her family often went downstairs to help with office work and filing after hours.
The bookcase hid the door to the hiding spot from the day workers.
Anne frequently wrote about how much she loved to read, but never had enough books.
A fire could start in the neighboring buildings and spread to the Annex.
Thieves would break into the Annex and discover the family after hours
The neighbors might overhear them on the weekends.
Any light or movement in the upstairs windows would give away their hiding space.
The industrial neighborhood might be targeted by German bombers.