The rate of disintegration of a fixed quantity of a radioactive element can be increased by

One mole of calcium phosphide on reaction with excess water gives

Ethyl chloride reacts with sodium ethoxide to form a compound $A$. Which of the following reactions also yields $A$?

Which one of the following is not a condensation polymer?

Consider the following statements $A$ and $B$ and identify the correct answer.
$A$. In an elastic collision,if a body suffers a head-on collision with another of the same mass at rest,then the first body comes to rest while the other starts moving with the velocity of the first one.
$B$. Two bodies of equal mass suffering a head-on elastic collision merely exchange their velocities.

The packing efficiency of the two-dimensional square unit cell shown below is ........... $\%$