$(a)$ Give an example of a combination reaction which is exothermic.
$(b)$ Identify the oxidizing agent and reducing agent in the following reaction:
$H_{2}S + Cl_{2} \rightarrow 2HCl + S$
$(c)$ Name the phenomenon due to which the taste and smell of oily food changes when kept for a long time in the open. Suggest one method to prevent it.

(N/A) An example of an exothermic combination reaction is the formation of water from hydrogen and oxygen: $2H_{2}(g) + O_{2}(g) \rightarrow 2H_{2}O(l) + \text{Heat}$.
$(b)$ In the reaction $H_{2}S + Cl_{2} \rightarrow 2HCl + S$:
- The oxidizing agent is $Cl_{2}$ because it gains electrons (or helps in the removal of hydrogen).
- The reducing agent is $H_{2}S$ because it loses hydrogen (or provides electrons).
$(c)$ The phenomenon is known as Rancidity. It can be prevented by storing food in airtight containers to prevent oxidation.