A:N2(g)+2O2(g)=2NO2(g)△H =+67.7kJ/mol
B:CaO(s)+CO2(g)==CaCO3(s)△H=-175.7kJ/mol
C:(NH4)2CO3(s)==NH4HCO3(s)+NH3(g)△H=+74.9kJ/mol
D:2H2(g)+O2(g)==2H2O(l) △H=-285.8kJ/mol
写出下列各题中x与y之间的关系式,并判断y是否为x的一次函数?是否为正比例函数?