已知充分燃烧a g乙炔气体时生成1 mol二氧化碳气体和液态水，并放出热量b kJ，则乙炔燃烧的热化学方程式正确的是

A．2C2H2(g)＋5O2(g)===4CO2(g)＋2H2O(l)ΔH＝－2b kJ·mol－1

B．C2H2(g)＋O2(g)===2CO2(g)＋H2O(l)ΔH＝＋2b kJ·mol－1

C．2C2H2(g)＋5O2(g)===4CO2(g)＋2H2O(l)ΔH＝－4b kJ·mol－1

D．2C2H2(g)＋5O2(g)===4CO2(g)＋2H2O(l)ΔH＝＋b kJ·mol－1