Everyday at school, Jo climbs a flight of 6 stairs. Joe can take the stairs 1,2, or 3 at a time. For example, Jo could climb 3, then 1, then 2. In how many ways can Jo climb the stairs?<spanclass=′latex−bold′>(A)</span>13<spanclass=′latex−bold′>(B)</span>18<spanclass=′latex−bold′>(C)</span>20<spanclass=′latex−bold′>(D)</span>22<spanclass=′latex−bold′>(E)</span>24