Biology

A biologist is testing the effects of water temperature, salinity, and oxygen concentration on the hatching rate of fish eggs. She sets up 27 tanks, each with a unique combination of the following conditions:

1. Water Temperature Levels: Low (15°C), Medium (20°C), High (25°C)

2. Salinity Levels: Low (5 ppt), Medium (15 ppt), High (30 ppt)

3. Oxygen Concentration Levels: Low (4 mg/L), Medium (7 mg/L), High (10 mg/L)

She records the percentage of eggs that hatch in each tank after 10 days.

What is the total number of independent variables in this experiment?