In the given graph,what does the horizontal line passing through the middle represent?

Which one of the following is $NOT$ a characteristic of a habitat?

Which feature of an elephant suggests that it is an animal of a hot climate?

Match the types of niche in Column-$I$ with their description in Column-$II$.

$i$. Habitat niche	$a$. In the absence of competition in the habitat
$ii$. Trophic niche	$b$. Position of organism in the environmental gradient
$iii$. Multidimensional niche	$c$. Physical space occupied by an organism
$iv$. Fundamental niche	$d$. Level of organism in the food chain

$A$ regulators are able to maintain homeostasis by means which ensure constant body temperature,constant osmotic concentration,etc. All $B$ and $C$ and a very few lower vertebrate and invertebrate species are indeed capable of such regulation (thermoregulation and osmoregulation).
Evolutionary biologists believe that the 'success' of mammals is largely due to their ability to maintain a constant body $D$ and thrive whether they live in Antarctica or in the Sahara desert.
Choose the correct option for $A, B, C$ and $D$.

How can seals survive in a polar climate where the temperature prevails below $0^{\circ} C$?