If $\overrightarrow a=\hat i-\hat k,\:\overrightarrow b=x\hat i+\hat j+(1-x)\hat k\:and\:\overrightarrow c=y\hat i+x\hat j+(1+x-y)\hat k$, then $[\overrightarrow a\:\overrightarrow b\:\overrightarrow c]$ depends on?

Question

$\begin{array}{1 1} Only \;x \\ Only \;y \\ neither\;x \;nor\;y \\ both\;x\;and\;y \end{array} $