A parametric curve defined by $x= \cos \left ( \dfrac{\Pi u}{2} \right ), y= \sin \left ( \dfrac{\Pi u}{2} \right )$ in the range $0 \leq u \leq 1$ is rotated about the $X$-axis by $360$ degrees. Area of the surface generated is

- $\dfrac{\Pi }{2} \\$
- $\pi \\$
- $2 \pi \\$
- $4 \pi$