It takes Mary 30 minutes to walk uphill 1 km from her home to school, but it takes her only 10 minutes to walk from school to home along the same route. What is her average speed, in km/hr, for the round trip?
<spanclass=′latex−bold′>(A)</span>3<spanclass=′latex−bold′>(B)</span>3.125<spanclass=′latex−bold′>(C)</span>3.5<spanclass=′latex−bold′>(D)</span>4<spanclass=′latex−bold′>(E)</span>4.5