Samples X and Y contain equal numbers of atoms at $t=0$. The half-life of X is $10$ days and that of Y is $20$ days. Which statements are correct?

1. AThe initial activity of X is twice that of Y.
2. BAfter $20$ days the number of undecayed atoms of X : Y is $1:2$.
3. CAfter $20$ days the activities of X and Y are equal.
4. DAfter $40$ days the activities of X and Y are equal.