I'll do A for you. They are all double displacement reactions. So recall: AB+CD--> CB+AD . Also, knowing the charges will help and make this so easy.
If you double the concentration of A, the rate is doubled. If you doubled the concentration of B, since the rate law has B^2, the rate is multiplied by 4. If you double both A & B's concentrations, then the rate would be multiplied by 2*4 = 8, which is the last option.
The density of the object is 19 g/cm³.
The mass of the object is 73 g.
The volume of the object is 3.8 cm³.
Density = mass/volume = 73 g/3.8 cm³ = 19 g/cm³
