Answer;
(3) Substance A is an element and substance Z is a compound.
Explanation;
-A compound is a substance formed when two or more chemical elements are chemically bonded together, such as water, carbon dioxide, methane, etc while an element is a pure chemical substance made of same type of atoms, such includes, oxygen, potassium, hydrogen, carbon, etc.
- Compounds can be broken
down or decomposed into simpler substances: elements cannot be broken down chemically into simpler substances .
Answer:

Recommended daily Amount (RDA) of magnesium is 410,000 μg/day.
Explanation:
There are 1000μg in 1 mg and 1000 mg in 1g
1 mg=1000μg
The Recommended daily Amount (RDA) is 410 mg/day of magnesium. Converting 410 mg/day into μg/day

It will become 410,000 μg/day
So Recommended daily Amount (RDA) of magnesium is 410,000 μg/day.
Answer: a: reactants Na-2 Cl-2
Products: Na-2 Cl-2;
b: reactants P-1 Cl-13 H-6 Products P-1 H-6 Cl-13
c: reactants P-4 H-12 O-16
Products H-12 P-4 O-16
Explanation: since these equations are balanced the atoms on of element on the reactants side will be same as the atoms of the same element of the product side
on the left side of the arrow