To solve this problem, let's assume the two-digit number as "10a + b" where 'a' is the tens digit and 'b' is the units digit.
The number obtained by interchanging the digits is "10b + a".
According to the given information, the average between these two numbers is 9. We can express this mathematically as:
(10a + b + 10b + a) / 2 = 9
Simplifying the equation, we get:
11a + 11b = 18
Dividing both sides by 11, we get:
a + b = 18/11
Since 'a' and 'b' represent integers, the sum of two integers cannot be a non-integer value. Therefore, it is not possible to determine the difference between the two digits of the number.
Hence, the correct answer is D. Cannot be determined.