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In a $\text{P}$ type silicon sample, the hole concentration is $2.25 \times 10^{15} / \mathrm{cm}^{3}$. If the intrinsic carrier concentration is $1.5 \times 10^{10} / \mathrm{cm}^{3}$, the electron concentration is

1. zero
2. $10^{10} / \mathrm{cm}^{3}$
3. $10^{5} / \mathrm{cm}^{3}$
4. $1.5 \times 10^{25} / \mathrm{cm}^{3}$.