已知H-H键能为436KJ/mol,H-N键能为391KJ/mol,根据化学方程式:N 2 +3H 2 =2NH 3 ΔH=-92.4KJ/mol,则N≡N键的键能是 [ ] A.431 KJ/mol B.946 KJ/mol C.649 KJ/mol D.869 KJ/mol