generally a linear function takes the form of:

v(x1,x2) = C1 * x1 + C2 * x2 + C3.

C's being "coefficients".

In your case, v(x1,x2) = 0x1 + x2 + 0

i.e. C1 = 0; C2 = 1; C3 = 0

suffices, where a result v > 0.5 indicates A and v <= 0.5 indicates B.