已知充分燃烧a g 乙炔气体时生成1mol二氧化碳气体和液态水，并放出热量bkJ，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)；△H = －4b KJ/mol |
| B．C2H2(g) + 5/2O2(g) ="=" 2CO2(g) + H2O(l)；△H =" 2b" KJ/mol |
| C．2C2H2(g) + 5O2(g) ="=" 4CO2(g) + 2H2O(l)；△H = －2b KJ/mol |
| D．2C2H2(g) + 5O2(g) ="=" 4CO2(g) + 2H2O(l)；△H =" b" KJ/mol |