My vote is for A.
difference between salaries is $5. Pick numbers...30 and 35 + 6% increase yields a difference of $5.30. You get the same exact value when you pick other numbers that are $5 apart (i.e 42 and 37). So the dollar difference will always be $5 (+change bue question asks about $ difference)
Not enough information. 4:3 ratio could mean that one person makes $12 and the other makes $9 or that one person makes $40 and the other $30. Difference in salary would vary widely then - not sufficient.
Interesting observation. Tried the same with $100 and $105 and it also came out to a $5.30 difference.