Dissolved Oxygen (DO) Is an Important Indicator of Water Quality

Dissolved oxygen (DO) is an important indicator of water quality, and oxygen levels that are too high or too low can harm aquatic ecosystems. A biologist noticed that fish were dying in a local lake and was concerned that the DO levels were too low to support aquatic life due to extremely high temperatures that summer. To measure the DO levels, she brought back several lake water samples to her laboratory to measure the DO concentration as a function of temperature. To do this, she heated several samples of lake water to various temperatures and then measured the DO concentration in each sample. Some samples were kept at the outdoors temperature.
Why were some samples kept outdoors?

A) To minimize researcher bias
B) To calibrate the thermometer
C) To allow a maximum amount of oxygen to dissolve before measurement
D) To act as a control group