In its natural condition, a soil sample has a mass of $1.980$ kg and a volume of $0.001 \: m^3$. After being completely dried in an oven, the mass of the sample is $1.800$ kg. Specific gravity G is $2.7$. Unit weight of water is $10 \: kN/m^3$. The degree of saturation of the soil is
1. $0.65$
2. $0.70$
3. $0.54$
4. $0.61$