What effect does the STC circuit have on the sea return on the PPI scope?

The STC (Sensitivity Time Control) circuit is designed to manage the radar receiver's sensitivity over time, particularly in relation to the range of targets being detected. When the STC circuit is operational, it typically reduces the gain of the radar receiver for returns that come from closer ranges while maintaining higher gain for returns from farther away.

In the context of decreasing sea return on the PPI (Plan Position Indicator) scope, the effect of the STC circuit is crucial. Sea return is often characterized by clutter returns from waves and the general surface of the water, which can mask or obscure the detection of actual targets (like other vessels or obstacles). By decreasing the sensitivity to returns that are close, the STC circuit minimizes the clutter from the sea surface. This is particularly valuable in enhancing the clarity of the display, allowing operators to see important targets more distinctly without being overwhelmed by the returns from the ocean itself.

Therefore, the correct answer highlights the role of the STC circuit in effectively managing and reducing unwanted sea clutter on the PPI scope, thus improving the operator's ability to discern significant targets amidst the background noise of sea returns.