In the following question, which of the given statement(s) are necessary for determining the answer.
What is the sum of the present ages of father and the son?
I. Father's present age is thrice son's present age
II. After 6 years the ratio of father's age to the son's age will be 12 ∶ 5
III. The difference between the father's age and the son's age is equal to twice the son's age.
1
I and III
2
II alone
3
II and either I or III
4
I and either II or III
5
None of these