Pyramid of energy in a pond ecosystem is always

Consider the following statements concerning the food chain:

(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 dear.

(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 $2-8$ trophic levels.

Which two of the statements are correct?

In an aquatic ecosystem, the trophic level equivalent to cows in grasslands is

Identify which one of the following is an e.g. of incomplete ecosystem

Or

Which of the following habitats is most unsuitable for primary productivity

Identify the correct type of food chain

Dead animal$\to$blow fly maggots $\to$ common frog $\to$snake