Consider the statements given below:
A. Discrete-time LTI system with impulse response $h[n] = \left(\frac{1}{2}\right)^n u[n-1]$ is causal
B. Response $(-2)^n u[n]$ may or may not be stable
C. Response $(-2)^n u[n]$ is unstable
D. Response $(-2)^n u[-n-1]$ is unstable