Which ecosystem is more stable?

Which one of the following is not a functional unit of an ecosystem?

Consider the following statements concerning food chains.
$A.$ Removal of $80\%$ tigers from an area resulted in greatly increased growth of vegetation.
$B.$ Removal of most of the carnivores resulted in an increased population of deers.
$C.$ The length of food chains is generally limited to $3-4$ trophic levels due to energy loss.
$D.$ The length of food chains may vary from $2$ to $8$ trophic levels.
Which two of the above statements are correct?

Herbivores are called

Which of the following is the correct sequence in a food chain?

Ecological succession on sand is known as: