There are 3 containers of equal capacity and all are completely filled with a mixture of acid and water. The ratio of acid and water in the three containers is 1 : 4, 3 : 7 and 2 : 3, respectively. If the contents of all the 3 containers are mixed together, what is the ratio of acid to water in the mixture?

A

3 : 10

B

9 : 14

C

3 : 7

D

9 : 7