Which of the following is not a breeding site for mosquitoes?

Fast-flowing rivers are typically not considered breeding sites for mosquitoes. Mosquitoes prefer locations where water is stagnant or has very slow movement, as these conditions allow them to lay eggs that can develop without the risk of being swept away. Stagnant water bodies, like flooded fields, containers that hold standing water, and marshy areas, create ideal environments for mosquito breeding due to the calm water that allows larvae to thrive.

In contrast, fast-flowing rivers experience conditions that are unsuitable for mosquito larvae. The current can disrupt the eggs and larvae, making it difficult for them to survive and develop. Therefore, while mosquitoes are highly adaptable and can be found in a variety of environments, fast-flowing rivers are not conducive to their reproduction, unlike the other options listed.