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 her home along the same route. What is her average speed, in km/hr, for the round trip?
(A) 3 (B) 3.125 (C) 3.5 (D) 4 (E) 4.5
ANSWER: A