XX Question 3, XXX same principle XX XXXXXXX as XXXXXXXX X. XX XXXX case, the specific XXXXXXX, γ, is the weight XXX unit volume, or in other words XXXXXXX XXXXXXXXXX by g. XXX can thus directly use XXX XXXXXXXX X=XX, XXX i still used P=XXXXXX later XXXXXXXXX ρgXX X.