$Rn$ decays into $Po$ by emitting an $\alpha$-particle with a half-life of $4 \text{ days}$. $A$ sample contains $6.4 \times 10^{10}$ atoms of $Rn$. After $12 \text{ days}$,the number of atoms of $Rn$ left in the sample will be:

• A
  $3.2 \times 10^{10}$
• B
  $0.53 \times 10^{10}$
• C
  $2.1 \times 10^{10}$
• D
  $0.8 \times 10^{10}$