$99 \%$ of a radioactive element will decay between

The activity of a radioactive substance can be represented by various units. Select the correct option.

In an old rock,the ratio of uranium to lead nuclei is $1:1$. The half-life of uranium is $4.5 \times 10^9$ years. If the rock initially contained only uranium nuclei,how old is the rock?

Define the decay constant and state its $SI$ unit.

$A$ radioactive sample is an $\alpha$-emitter with a half-life of $138.6$ days. $A$ student observes its activity to be $2000$ disintegrations per second. The number of radioactive nuclei for this given activity is:

The plot of the number $(N)$ of undecayed atoms versus activity $(A)$ of a radioactive substance is