13. An aqueous solution containing compounds A and B shows optical activity. A and B are stereoisomers. Which of the following possibilities cannot be correct?
(a) A has two centers, but B does not have any because it has a symmetry plane.
(b) A and Bare enatiomers.
(c) A and B are distereomers.
(d) A and B are not present in equal amounts.

Optically inactive solution  can be due to
1) Racemic mixture  (both enantiomers  present in equal amount)
2)Compound is meso form
In our question:
a) A and B are streoisomers so it is not possible the A has chiral centre but B does not have any
b) It is possible and both of them are present in unequal amount.
c)Yes ,that can be feasible .
d) That is also correct because then solution is not a racemic mixture.