**Core Concept**
The thermoregulatory system is a complex process that involves the hypothalamus, which acts as the body's temperature sensor, and various physiological responses to maintain a stable body temperature. The gain of the thermoregulatory system refers to its sensitivity and ability to respond to changes in temperature, with higher gain indicating a more effective response.

**Why the Correct Answer is Right**
The gain of the thermoregulatory system can be calculated using the formula: Gain = (ΔT_setpoint / ΔT_actual) - 1, where ΔT_setpoint is the desired change in temperature and ΔT_actual is the actual change in temperature. In this case, the body temperature was expected to decrease from 37°C to 20°C, but it only decreased to 36.5°C. This means that the actual change in temperature (ΔT_actual) was 1.5°C (37°C - 36.5°C), which is less than the expected change (ΔT_setpoint = 7°C, from 37°C to 30°C). Therefore, the gain of the thermoregulatory system is calculated as: Gain = (7°C / 1.5°C) - 1 = 3.33.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided.
**Option B:** This option is not provided.
**Option C:** This option is not provided.
**Option D:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
The gain of the thermoregulatory system is an important concept in understanding how the body responds to changes in temperature. A high gain indicates a more effective response, while a low gain suggests a less effective response, which can lead to hypothermia or hyperthermia.

**Correct Answer:** **C**.