Which one of the following types of organisms occupy more than one trophic level in a pond ecosystem?

How many statements are correct?
$(a)$ Some animals like cockroaches,crows,etc.,are omnivores.
$(b)$ Detritus food chain $(DFC)$ begins with dead organic matter.
$(c)$ Each trophic level has a certain mass of living material at a particular time called as standing crop.
$(d)$ Species composition and stratification are two main structural features of an ecosystem.

Which of the following are the essential sources for releasing $CO_{2}$ into the atmosphere?
$I.$ Burning of wood
$II.$ Volcanic activity
$III.$ Combustion of organic matter
$IV.$ Fossil fuels
Choose the correct option.

Omnivores feed on members of which trophic levels?

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

In the phosphorus cycle, phosphate becomes available by weathering of rocks first to