Given an Equation Such as X + Y = 18

Given an equation such as x + y = 18, children determine that the ordered pair (9, 9) is not possible. The most likely reason for this is:

A) they don't think that x and y can be the same number, since the variables are different.
B) they are incapable of thinking of this pair or any other ordered pair because they are not yet thinking abstractly.
C) they think that the ordered pair (10, 8) is the only possible answer.
D) they don't understand the role of the equal sign.