Which type of reaction occurs without being started by some outside force?
combustion reaction
physical reaction
spontaneous reaction
activation energy reaction

Option (c) is the correct answer.

Explanation:

A spontaneous reaction is a reaction which occurs on its own without any intervention or outside force helping in initiation of the reaction.

A combustion reaction is the reaction in which a compound reacts with oxygen atom resulting in the formation of water and carbon dioxide gas.

A physical reaction is a reaction in which there occurs no change in chemical composition of a substance.

Activation energy is the reaction in which minimum amount of energy is required to carry out a reaction. More is the activation energy of a reaction slower will be the rate of reaction, therefore a catalyst is required to increase the rate of reaction.

Thus, we can conclude that spontaneous reaction is the type of reaction that  occurs without being started by some outside force.

acid rain does not usually kill trees directly. instead, it is more likely to weaken the trees by damaging their leaves, limiting the nutrients available to them, or poisoning them with toxic substances slowly released from the soil. the main atmospheric pollutants that affect trees are nitrates and sulphates.

the product of the pressure and volume is also constant