Subject: mathqa
Question: city a and city b are 140 miles apart . train c departs city a , heading towards city b , at 4 : 00 and travels at 40 miles per hour . train d departs city b , heading towards city a , at 4 : 40 and travels at 20 miles per hour . the trains travel on parallel tracks . at what time do the two trains meet ?
Option: 
A: 5 : 30
B: 7 : 00
C: 6 : 40
D: 6 : 30
Answer: C