Natural gas is a mixture of .........?

$X-$ It is very important for the process of evolution.
$Y-$ Crossing over is the exchange of genetic material between two homologous chromosomes.

In printed scientific names, only the . . . . . . is capitalized.

What is common between parrot,Platypus and kangaroo?

The number of unit cells present in $39 \ g$ of potassium if it crystallizes as body-centered cubic $(BCC)$ is ($N = \text{Avogadro number}$,$\text{At. wt. of potassium} = 39$).

Four charges equal to $-Q$ are placed at the four corners of a square and a charge $q$ is at its centre. If the system is in equilibrium,the value of $q$ is