Borrow two books at will. There are three situations: (popular science books, story books), (popular science books, pictorial) and (story books, pictorial). There are three situations in which you can borrow books at will;
A * * * is 3+3=6 (species);
In six cases, six drawers were constructed,
6+ 1=7 (pieces),
At least seven students must borrow books to ensure that two of them must borrow books of the same category.