Subject: mathqa
Question: the ratio between the present ages of a and b is 7 : 3 respectively . the ratio between a ' s age 4 years ago and b ' s age 4 years hence is 1 : 1 . what is the ratio between a ' s age 4 years hence and b ' s age 4 years ago ?
Option: 
A: 9 : 1
B: 9 : 2
C: 3 : 4
D: 3 : 0
Answer: A