数据499,500,501,500的中位数是____.
①N2(g)+O2(g)⇌2NO(g)△H1=a kJ•mol﹣1
②2NO(g)+O2(g)⇌2NO2(g)△H2=b kJ•mol﹣1
③CO(g)+ 12 O2(g)⇌CO2(g)△H3=c kJ•mol﹣1
④2CO(g)+2NO(g)⇌N2(g)+2CO2(g)△H4
请回答下列问题:
hard的反义词是____.