How does a Moving Target Indicator (MTI) system suppress ground clutter (such as buildings and hills) on a primary radar display?

1. By filtering out returned signals that exhibit no phase/Doppler frequency shift over successive pulses.
2. By reducing the transmitter power output when scanning near the horizon.
3. By manually masking out fixed areas of the screen where hills are known to exist.
4. By rejecting signals that have a weak signal-to-noise ratio.