Solve this following
Question:

If $A=\left(\begin{array}{lll}1 & \lambda & 2 \\ 1 & 2 & 5 \\ 2 & 1 & 1\end{array}\right)$ is not invertible then $\lambda=$ ?

A. 2

B. 1

C. $-1$

D. 0

Solution: