Soru

Biologists conducted an experiment to investigate the effects of heavy metal contamination on the photosynthetic activity of the freshwater alga *Chlorella vulgaris*. Four identical culture flasks were prepared, each containing $100\text{ mL}$ of standard growth medium and an initial cell density of $1.0 \times 10^5\text{ cells/mL}$:

- Flask 1: Standard growth medium with no added heavy metal ions.
- Flask 2: Standard growth medium containing $5.0\text{ mg/L}$ of copper ions ($\text{Cu}^{2+}$).
- Flask 3: Standard growth medium containing $5.0\text{ mg/L}$ of lead ions ($\text{Pb}^{2+}$).
- Flask 4: Standard growth medium containing $5.0\text{ mg/L}$ of cadmium ions ($\text{Cd}^{2+}$).

All four flasks were kept at $25^\circ\text{C}$ under continuous illumination for 48 hours. Dissolved oxygen ($\text{DO}$) concentration was then measured to quantify photosynthetic output.

Which flask served as the control group in this experiment, and what was its primary purpose?