Subject: mathqa
Question: jack and jill are marathon runners . jack can finish a marathon ( 42 km ) in 3.5 hours and jill can run a marathon in 4.2 hours . what is the ratio of their average running speed ? ( jack : jill )
Option: 
A: 6 / 5
B: can not be determined
C: 15 / 14
D: 4 / 5
Answer: A