已知N≡N键的键能是a kJ﹒mol-1，H－H键的键能是b kJ﹒mol-1，N－H键的键能是c kJ﹒mol-1，则N2(g) + 3 H2(g) =" 2" NH3(g)的△H为( )kJ﹒mol-1
| A．3c–a-b | B．a+b-3c | C．a+3b-2c | D．a+3b-6c |
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