A can contains a mixture of two liquids A and B in the ratio 7:5. When 9 litres of the mixture are drawn and replaced by the same amount of liquid B, the ratio of A and B becomes 7:9. How many litres of liquid A was contained in the can initially?
<spanclass=′latex−bold′>(A)</span>18<spanclass=′latex−bold′>(B)</span>19<spanclass=′latex−bold′>(C)</span>20<spanclass=′latex−bold′>(D)</span>None of the above