Maximum contribution of $O_2$ is from

Identify $X$ from the figure:

Select the incorrect statement from the following.

The given food web contains some missing organisms $A, B, C$ and $D$. Identify these organisms and select the correct answer.

Consider the following statements about food chain:
$I.$ The transfer of energy from producers to top consumers through a series of organisms is called a food chain.
$II.$ $A$ food chain is always straight and proceeds in a progressive straight line.
$III.$ In a food chain,there is a unidirectional flow of energy from the sun to producers and subsequently to a series of different types of consumers.
Which of the statements given above are correct?

The first link in any food chain is a green plant because