An excitation is applied to a system at $$t = T$$ and its response is zero for $$ - \infty < t < T$$. Such a system is a

The pole-zero pattern of a certain filter is shown in the Fig. The filter must be of the following type.

A signal has frequency components from 300 Hz to 1.8 KHz. The minimum possible rate at which the signal has to be sampled is ______ (fill in the blank).