What is the average of a, b, c, d?
(I) a + b + c = d + 15.
(II) 13 - c - 3d = a + b.

if the statement (I) alone is sufficient to answer the question, but (II) does not answer the question.

if the statement {II) alone is sufficient to answer the question but (I) does not answer the question.

if both the statements (I) and (II) are sufficient to answer the question but neither statement alone is sufficient.

if both the statements (I) and (II) together are not sufficient to answer the question and additional data is required.