Final answer:
The collective noun for a group of cobras is a quiver. If you were Katniss Everdeen for Halloween, you probably had to buy a bow and arrow as part of your costume.
Step-by-step explanation:
The collective noun for a group of cobras is quiver. It is called a quiver of cobras. This term originated from the characteristic behavior of cobras when they raise their bodies and flatten their necks, making it look like they are trembling or quivering.
If you were Katniss Everdeen for Halloween, you probably had to buy a bow and arrow as part of your costume, which is a major characteristic of her character in The Hunger Games trilogy. The mention of buying a bow and arrow is a hint to the collective noun 'quiver'.