Anyone can borrow books from this library _______ he keeps them clean and returns them in time.

1. A.
   even if
2. B.
   unless
3. C.
   so that
4. D.
   as long as

Anyone can borrow books from this library _______ he keeps them clean and returns them in time.

A. even if                 B. unless                  C. so that                          D. as long as