Answer: Option (a) is the correct answer.
Explanation:
A decomposition reaction is defined as a reaction where a compound dissociates into two or more atoms.
For example, 
Whereas a chemical reaction in which two reactants combine together to result in the formation of a compound is known as a synthesis reaction.
For example,
is a synthesis reaction.
Thus, we can conclude that the statement chemical reaction A, because the reactant is a compound identifies the decomposition reaction and describes a substance is involved.
Answer:
rise,cool..................
Isotopes of any given factor all incorporate the equal variety of protons, so they have the identical atomic wide variety (for example, the atomic wide variety of helium is usually 2). Isotopes of a given factor include exceptional numbers of neutrons, therefore, special isotopes have special mass numbers.
Answer:
Mass is the quantity of an object.
Weight is the force with which the object is attached towards the earth.
Hope it helps