What is a half$-$life?

Question $5$ options:

A$)$

A game.

B$)$

Seriously, Dr$.$ White?

C$)$

It is the time it takes for $1/2$ of the material to undergo radioactive decay.

D$)$

It is the time needed until $1/2$ of the radiation is gone.

E$)$

It is the time it takes for $1/2$ of the material to decompose.