Maximum contribution of $O_2$ is from

Which of the following statements are correct?
$(a)$ Energy flow from producers to consumers is unidirectional
$(b)$ Energy pyramid can never be inverted
$(c)$ Transfer of energy follows the $10\%$ law
Choose the correct answer from the options given below:

$A$: Estuary constitutes one of the most productive ecosystems.
$R$: It represents a transition zone between two populations of a species.

Identify the correct sequence of development of a stable community (climax community) starting from a new area (primary succession).

The order of organisms in an aquatic food chain is:

Which of the following characteristics of an ecosystem is naturally provided to human society?