How Can Water Loss from the Body Be Quickly Estimated

How can water loss from the body be quickly estimated at the end of a physical activity?

A) Multiply body weight (kg) by 1% to determine liters of water loss
B) Multiply duration of activity (min) by body weight (kg) to determine mL of water loss
C) Subtract air temperature (°F) from body weight (lbs) and then multiply by 5 to determine mL of water loss
D) Take the difference in body weight (lbs) before and after the event and multiply by 2 to determine cups of water loss