The driving force of any ecosystem is

Which one of the following components of an ecosystem comes from outside?

Assertion $A$: Heat energy is available for the functioning of a system.
Reason $R$: Some amount of energy is lost during energy transformation.
Which option is correct for Assertion $A$ and Reason $R$?

If $20 \, J$ of energy is trapped at the producer level,then how much energy will be available to the peacock in the following food chain (in $, J$)?
Plant $\to$ Rat $\to$ Snake $\to$ Peacock

The main source of energy in an ecosystem is .........

Which one of the following organisms is likely to show the highest concentration of $DDT$,once it has been introduced into the ecosystem?