Let $N _ { \beta }$ be the number of $\beta$ particles emitted by 1 gram of $\mathrm { Na } ^ { 24 }$ radioactive nuclei having a half life of 15 h. In 7.5 h, the number $N _ { \beta }$ is close to $\left[ N _ { \mathrm { A } } = 6.023 \times 10 ^ { 23 } \mathrm {~mole} ^ { - 1 } \right]$\\
(1) $1.75 \times 10 ^ { 22 }$\\
(2) $6.2 \times 10 ^ { 21 }$\\
(3) $7.5 \times 10 ^ { 21 }$\\
(4) $1.25 \times 10 ^ { 22 }$