Answer: The precipitate formed is 
Explanation:
A double displacement reaction is one in which exchange of ions take place. The salts which are soluble in water are designated by symbol (aq) and those which are insoluble in water and remain in solid or precipitated form are represented by (s) after their chemical formulas.
A double displacement reaction in which one of the product is formed as a solid is called as precipitation reaction.
The balanced chemical equation is:

Answer:
D. VISCOSITY can be defined as a substance's ability to resist flow.
Explanation:
This is the statement that correctly describe the term viscosity. Viscosity explains why substances or fluids resist flow. It shows that internal friction or resistance to flow revealed by a substance. A substance with high viscosity or internal friction resist flow and one with low viscosity flow easily. Viscosity of a substance change base on various factors affecting the substance such as pressure, temperature, surface area. Substances like water has low viscosity explaining why it flows easily.
n=20 mol
(NH)4 SO4
Atomic masses :
N- 14
H- 1
S- 32
O- 16
Therefore M= 14×2 + 1×8 + 32 + 16×4
= 132
m= nM
= 20×132
= 2640g