Generally the food chain has how many trophic levels

$...A...$ is required for higher primary productivity. $...B...$ have the lowest primary productivity as the soil is deficient in moisture.

Choose the correct option for $\mathrm{A}$ and $\mathrm{B}$

A wolf has just eaten a lamb when tiger saw the wolf attacked it and consumed it. The tiger is in ecological terms

Consider the following statements about food chain

$I.$ The transfer of energy from producers to top consumers through a series of organisms is called food chain

$II.$ A food chain is always straight and proceeds in a progressive straight line

$III.$ In a food chain, there is unidirectional flow of energy from sun to producers and subsequently to series of different types of consumers

Which of the statements given above are correct?

Which group of living organisms given below is a link in food chain between green plants and hawk

Consider the following four statements $(A-D)$ and select the correct option stating which ones are true $(T)$ and
which ones are false $(F)$.
Statements :
$(A)$ Vertical distribution of different species occupying different levels in a community is called stratification.
$(B)$ Net primary productivity minus respiration losses is the gross primary productivity.
$(C)$ Rate of biomass production is called decomposition
$(D)$ Annual net primary productivity of oceans is $55$ billion tons