Explanation: The symptom described—a blank spot in the center of the CRT where no target video appears, and this blank spot shrinks as the range scale increases—is a classic indication of an overly aggressive or misadjusted **Sensitivity Time Control (STC)**. STC is designed to reduce the gain of the radar receiver for echoes returning from very short ranges, typically to suppress strong returns from sea clutter, rain clutter, or the radar's own antenna structures near the vessel. As the range increases, the STC gradually restores the receiver's full gain. If the STC is set too high or is malfunctioning, it will suppress all targets, including legitimate ones, at short ranges, creating the observed blank spot. As you increase the range scale, the point at which the STC's suppression effect diminishes moves further out, making the blank spot appear to shrink in diameter relative to the overall display. * **A) TUNE:** An improperly tuned radar would generally result in an overall weak or noisy display, or no display at all, not a selective blanking of close-in targets that diminishes with range. * **C) Anti-Clutter Rain (ACR):** Also known as Fast Time Constant (FTC) in some systems, ACR helps distinguish targets from rain clutter by emphasizing the leading edge of return pulses. Its misadjustment would affect target visibility within rain, not create a range-dependent blanking effect from the center. * **D) False Target Elimination (FTE):** This control (or feature) typically identifies and suppresses specific types of false echoes, such as those from side lobes or multiple reflections, rather than causing a general suppression of all close-in returns.