A student did an experiment on the cycling of environmental gases. She placed water and bromthymol blue in each of four test tubes. No other items were placed in tube 1. She placed a snail in tube 2, an aquatic(elodea) in tube 3 and both a snail and elodea in tube 4. Then she stoppered the tubes and placed them in bright light for 24 hours. The function of tube 1 in this experiment is to
1. Detect the presence of glucose
2. Determine the amount of gases in the water
3. Demonstrate the transparency of the solution
4. Serve as a control