The number of vectors $\overrightarrow b$ of unit length which are $\perp$ to $\overrightarrow a=\hat i+2\hat j\:\:and\:\:\overrightarrow c=\hat j+2\hat k$ is?

Question

$\begin{array}{1 1} 1 \\ 2 \\ 4 \\ Infinite\;vectors \end{array}$