13. A rope of negligible mass supports a block that weighs 30 N , as shown above. The breaking strength of the rope is 50 N . The largest acceleration that can be given to the block by pulling up on it with the rope without breaking the rope is most nearly
(A) 6.7m/s2
(B) 10m/s2
(C) 16.7m/s2
(D) 26.7m/s2