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Consider the sequence $x[n] = a^nu[n] + b^nu[n]$, where $u[n]$ denotes the unit-step sequence and $0<\mid a \mid < \mid b \mid<1$. The region of convergence (ROC) of the $Z$-transform of $x[n]$ is

- $\mid Z \mid > \mid a \mid$
- $\mid Z \mid > \mid b \mid$
- $\mid Z \mid < \mid a \mid$
- $\mid a \mid < \mid Z \mid < \mid b \mid$