For which value of kkk is the matrix D=(k22k)D = \begin{pmatrix} k & 2 \\ 2 & k \end{pmatrix}D=(k22k) singular?
k=2k = 2k=2 only
k=−2k = -2k=−2 only
k=±2k = \pm 2k=±2
k=0k = 0k=0