The op-amp shown in the figure has a finite gain $A = 1000$ and an infinite input resistance. A step-voltage $V_{i} = 1 mV$ is applied at the input at time $t = 0$ as shown. Assuming that the operational amplifier is not saturated, the time constant (in millisecond) of the output voltage $V_{0}$ is
1. $1001$
2. $101$
3. $11$
4. $1$