$A$ relationship in which a sea anemone is attached to the shell of a hermit crab is called:

Fill in the blanks:

Species $A$	Species $B$	Type of Interaction	Example
$+$	$-$	$(a)$	$(b)$
$+$	$+$	$(c)$	$(d)$
$+$	$(e)$	Commensalism	$(f)$

Study the following four statements $(1-4)$ and select the two which are correct:
$(1)$ Lion eating deer and sparrow feeding on grain are ecologically similar in being consumers.
$(2)$ The predator starfish $Pisaster$ helps in maintaining species diversity of some invertebrates.
$(3)$ Predators often lead to the extinction of prey species.
$(4)$ Production of chemicals such as nicotine and strychnine by plants is a metabolic disorder.
Which two statements are correct?

Choose the correct group for commensalism from the following:
$(a)$ The cattle egret and grazing cattle
$(b)$ Barnacle balanus and Barnacle chathamalus
$(c)$ Goat and Abingdon turtle from Galapagos island
$(d)$ Barnacles growing on the back of whale

Match the following population interactions with their respective signs and descriptions:

Interaction Type	Sign	Description
$(a)$ Parasitism	$(i) -, 0$	$(A)$ Both get benefitted
$(b)$ Amensalism	$(ii) -, -$	$(B)$ One gets harmed,other has no effect
$(c)$ Competition	$(iii) +, -$	$(C)$ Both get harmed
$(d)$ Mutualism	$(iv) +, +$	$(D)$ One is harmed and second is benefited

What is the term for an interaction where both organisms benefit and are essential for each other's survival?