If x,y and z are Mulliken electronegativity, ionisation potential and electron-affinity respectively. Then the electron affinity (z) in the terms of electronegativity (x) and ionisation potential (y) will be:
A. `z = (x+y)/(2)`
B. `z = (x-y)/(2)`
C. `z =(x^(2)-y^(2))/(2)`
D. `z = 2x - y`
