The standard enthalpy of formation of NH₃ is -46.0 kJ/mol. If bond enthalpy of H₂ is -436 kJ/mol and that of N₂ is -712 kJ/mol, the average bond enthalpy of N-H bond in NH₃ is:
a) -310 kJ/mol
b) -420 kJ/mol
c) -510 kJ/mol
d) -630 kJ/mol