Data Sufficiency

Hi folks - really need your help. When both the options of a DS question are sufficient but lead to different answers, should we choose D (each alone is sufficient) or E (not sufficient)?

For example, here is a question and its solution from Aristotle:
5. Five people - Adam, Bob, Craig, Daniel and Evan - are of different ages. Daniel is
younger than both Adam and Craig. Craig is younger than Bob but older than Evan.
Who among the five is the oldest?
(1) The average age of Adam and Bob is less than the average age of Craig and
Evan.
(2) The average age of Bob and Craig is less than the average age of Adam and
Evan.

Explanation:
Let the ages of Adam, Bob, Craig, Daniel and Evan be A, B, C, D and E.
Now D < A, C
E < C < B
Statement (1)
(A+B)/2 < (C+E)/2
A + B < C + E
But, B > C > E (given)
Therefore A has to be less than E to make the equality in Statement (1) true.
Hence D < A < E < C < B.
B is oldest; SUFFICIENT
Statement (2)
(B+C) / 2 < (A+E) / 2
B + C < A + E
But, B > C > E (given)
Therefore A has to be greater than B to make the equality in Statement (2) true.
Hence D < E < C < B < A.
A is oldest; SUFFICIENT
The correct answer is D;
each statement alone is sufficient.

Aristotle says you should choose D but I'm not convinced. Is there any official stance of the GMAT on this?

Thanks a ton in advance!