Which statement correctly defines a negative integer exponent for a nonzero base $a$ and positive integer $n$?

• A. $a^{-n}$ gives a negative result when $n$ is odd, and positive when $n$ is even.
• B. $a^{-n}=\frac{1}{a^n}$
• C. $a^{-n}=a^n$
• D. $a^{-n}=(-a{)}^n$

Answer: (B)