设 N_A为阿伏加德罗常数的数值，下列说法正确的是

A．N_A个CH₄分子的质量为16g/mol

B．1mol/LNaCl溶液含有N_A个Na⁺

C．等物质的量的N₂和CO所含分子数相同

D．22.4LCO₂中含有 N_A个CO₂分子

_____ a house five years ago, I would live a much easier life now. [ ]

A. Should if I buy 　　　　　

B. Should I buy 　　

C. Had if I bought 　　　　　

D. Had I bought