Solution: Let the slant height of the cone be $l$. According to the property that the arc length of a semicircle equals the circumference of the cone's base, we have $\pi l = 2 \sqrt { 2 } \pi$, so $l = 2 \sqrt { 2 }$.
\section*{3. B}
Solution: Let the slant height of the cone be $l$. According to the property that the arc length of a semicircle equals the circumference of the cone's base, we have $\pi l = 2 \sqrt { 2 } \pi$, so $l = 2 \sqrt { 2 }$.