**Core Concept**
Normal menstrual flow refers to the average volume of menstrual blood lost during a regular menstrual cycle. It is essential to understand the physiological mechanisms regulating menstrual flow to determine the correct answer.

**Why the Correct Answer is Right**
The average menstrual flow during normal menses is approximately 30-40 milliliters (mL) per day. This is due to the shedding of the endometrium, which is the lining of the uterus, under the influence of progesterone withdrawal. During the luteal phase, progesterone levels surge, causing the endometrium to thicken and prepare for a potential pregnancy. However, if pregnancy does not occur, progesterone levels drop, leading to the breakdown and shedding of the endometrium, resulting in menstrual bleeding.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is likely to be incorrect as it is too low. Menstrual flow is typically more than 20 mL per day.
* **Option B:** This option is likely to be incorrect as it is too high. While some women may experience heavy menstrual bleeding, the average flow is not typically more than 60 mL per day.

**Clinical Pearl / High-Yield Fact**
It's essential to note that menstrual flow can vary significantly among individuals, and some women may experience heavy or light bleeding. Heavy menstrual bleeding is defined as a flow exceeding 80 mL per day.

**Correct Answer: C. 30-40 mL.**