Letter B is incorrect because talking about the sea creatures <em>all over the world</em> would take longer than 500 words.
Letter D is also incorrect because a <em>detailed</em> study cannot be written in only one page.
Letter E is also incorrect because <em>the past century</em> is a very long period to be analyzed in such a short essay.
On the other hand, <u>letters A and C</u> are correct because their extension can be limited more easily than the others, specially when talking about personal information or a short period of time such as elementary school.
