List the marital status of customers within the age of 40 to 60 that has the highest income among the group.
age of 40 to 60 refers to age BETWEEN 40 AND 60; the highest income refers to MAX(INCOME_K);
SELECT T1.MARITAL_STATUS FROM Customers AS T1 INNER JOIN Demog AS T2 ON T1.GEOID = T2.GEOID WHERE T1.age >= 40 AND T1.age <= 60 ORDER BY T2.INCOME_K DESC LIMIT 1