将固体NH4I置于密闭容器中,在某温度下发生下列反应:NH4I(s)⇌NH3(g)+HI(g);2HI(g)⇌H2(g)+I2(g)。当达到平衡时:C(H2)=0.5mol/L,c(HI)=4mol/L,则NH3的浓度为( )。A.B.4mol/LC.D.5mol/L