Problems on Numbers - General Aptitude

Each of the questions given below consists of a question followed by three statements. You have to study the question and the statements and decide which of the statement(s) is/are necessary to answer the question.
Q1:

What is the two-digit number? I. Sum of the digits is 7. II. Difference between the number and the number obtained by interchanging the digits is 9. III. Digit in the ten's place is bigger than the digit in the unit's place by 1.

A I and II only

B II and III only

C I and III only

D All I, II and III

E None of these

ANS:E - None of these

Let the tens and units digit be x and y respectively.   I. x + y = 7.  II. (10x + y) - (10y + x) = 9        x - y = 1. III. x - y = 1. Thus, I and II as well as I and III give the answer.  Correct answer is (E).