A sample of radioactive element containing $4 \times10^{16}$ active nuclei. Half life of element is 10 days, then number of decayed nuclei after 30 days : -

Question

( A ) $1\times 10^{16}$
( B ) $3.5\times 10^{16}$
( C ) $0.5 \times 10^{16}$
( D ) $2 \times 10^{16}$