Question: Which of these chemical equations demonstrates the law of conservation of mass as it applies to the combustion of ethane?

  Options: 
 A. C_{2}H_{6} + 3O_{2} -> 2CO_{2} + 3H_{2}O
 B. C_{2}H_{6} + 5O_{2} -> 2CO_{2} + 3H_{2}O
 C. 2C_{2}H_{6} + 5O_{2} -> 4CO_{2} + 4H_{2}O
 D. 2C_{2}H_{6} + 7O_{2} -> 4CO_{2} + 6H_{2}O 
Answer: D