In the given figure,what does the part labeled '$a$' represent?

The lungs are covered by which of the following?

If $ab \neq 0$ and the sum of the coefficients of $x^7$ and $x^4$ in the expansion of $\left(\frac{x^2}{a}-\frac{b}{x}\right)^{11}$ is $0$,then

$A$ ball $P$ collides with another identical ball $Q$ at rest. For what value of coefficient of restitution $e$,will the velocity of ball $Q$ become two times that of ball $P$ after collision?

If $\frac{ax - 1}{(1 - x + x^2)(2 + x)} = \frac{x}{1 - x + x^2} - \frac{1}{2 + x}$,then $a = $

Identify the blank spaces $a, b, c$ and $d$ given in the following table and select the correct answer.

Type of microbes	Scientific name	Commercial product
Bacterium	$a$	Clot buster
$b$	Aspergillus niger	Citric acid
Fungus	Trichoderma polysporum	$c$
Bacterium	$d$	Butyric acid