Because a binary linear equation containing f(x) has at most two f(x), one f(x) corresponds to at most two x's, and these two x's must be symmetrical about the symmetry axis. AB has only two x's, so it must be symmetrical about a straight line. The four numbers 1, 2, 3 and 4 in C are symmetric about X=2.5, but the four numbers in D are not symmetric about a certain number, so D must not be the solution set of this equation.