Explanation:
Reaction rate is a measure of speed of a chemical reaction.
Usually, based on rates, reactions can be divided into two parts:
- Slow reactions which usually takes a long time.
- Fast reactions which takes place completely in seconds or microseconds.
Reaction rate can be defined as a change in the concentration of a reactant or product per unit time:
The unit is mol dm⁻³ s⁻¹
Reaction rate = 
The measurement of the reaction rate is based on the disappearance of a reactant or appearance of a product with increasing reaction time.
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The boiling point depends on the strength of the intermolecular forces holding the molecules together. Greater the force, higher is the boiling point.
The intermolecular force increases in the order shown below:
ion-ion > H-bonding > dipole-dipole > London dispersion
CH3CN is a polar molecule with strong dipole-dipole forces
CH3CH2CH3 is non-polar held by london dispersion
Ar exist as a gas. It will have a lowest boiling point
Thus the order of decreasing b.pt is:
CH3CN > CH3CH2CH3 > Ar
I believe the correct answer to your question is Brittle. Hope I'm correct let me know.
Combustion is burning and that usually produces carbon in the reaction adding it to the atmosphere