Explanation: The Sensitivity Time Control (STC) circuit, often used in radar systems, manages the receiver's gain immediately after a pulse transmission. Its primary function is to prevent the receiver from being overloaded by very strong echoes from nearby objects. By temporarily *decreasing* the receiver's sensitivity (gain) right after the transmit pulse, STC effectively attenuates strong returns from close-range targets. The sensitivity then gradually increases over time, allowing weaker echoes from more distant objects to be detected without being masked by the strong close-in signals. Therefore, it decreases sensitivity for close objects. Options B and C are incorrect because STC reduces, not increases, sensitivity for close objects, and while it *enables* detection of distant objects, its fundamental control action is the initial decrease in sensitivity. Option D is incorrect because STC controls the *receiver's* sensitivity, not the transmitter's.