For which kkk is the matrix A=(11112413k)A = \begin{pmatrix} 1 & 1 & 1 \\ 1 & 2 & 4 \\ 1 & 3 & k \end{pmatrix}A=11112314k singular?
k=6k = 6k=6
k=7k = 7k=7
k=8k = 8k=8
k=9k = 9k=9