标准状态下，气态分子断开1 mol化学键的焓变称为键焓。已知H—H、H—O和O===O键的键焓ΔH分别为436 kJ·mol^－1、463 kJ·mol^－1和495 kJ·mol^－1。下列热化学方程式正确的是

A．H2O(g)===H2＋1/2O2(g)ΔH＝－485 kJ·mol^－1

B．H2O(g)===H2(g)＋1/2O2(g) ΔH＝＋485 kJ·mol^－1

C．2H2(g)＋O2(g)===2H2O(g) ΔH＝＋485 kJ·mol^－1

D．2H2(g)＋O2(g)===2H2O(g) ΔH＝－485 kJ·mol^－1