标准状况下，气态分子断开1mol化学键的焓变称为键焓。已知H—H，H—O和O=O键的键焓△H分别为436kJ/mol，463kJ/mol，495kJ/mol，下列热化学方程式正确的是

A．H2O(g)=H2(g)+1/2O2(g) △H =-485kJ/mol

B．H2O(g)=H2(g)+1/2O2(g) △H =+485kJ/mol

C．2H2(g)+O2(g)=2H2O(g) △H =+485kJ/mol

D．2H2(g)+O2(g)=2H2O(g) △H =-485kJ/mol