Question

### Passage

An environmental scientist investigated the efficiency of sunflower plants (*Helianthus annuus*) in phytoremediation—the process of using living plants to remove heavy metals from contaminated soil. Sunflowers were grown in pots containing soil with a baseline lead ($Pb^{2+}$) concentration of $500\text{ mg/kg}$ under controlled greenhouse conditions. The scientist set up several experimental groups to evaluate how two soil amendments—biochar and arbuscular mycorrhizal fungi (AMF)—affect the rate of $Pb^{2+}$ uptake by the plants.

The experimental groups were designed as follows:
* Group A: Soil with $0\text{ mg/kg}$ of $Pb^{2+}$, no biochar, no AMF.
* Group B: Soil with $500\text{ mg/kg}$ of $Pb^{2+}$, no biochar, no AMF.
* Group C: Soil with $500\text{ mg/kg}$ of $Pb^{2+}$, biochar added ($5\%\text{ w/w}$), no AMF.
* Group D: Soil with $500\text{ mg/kg}$ of $Pb^{2+}$, no biochar, AMF added.
* Group E: Soil with $500\text{ mg/kg}$ of $Pb^{2+}$, biochar added ($5\%\text{ w/w}$), AMF added.

All groups were watered daily with $100\text{ mL}$ of distilled water and kept at a constant temperature of $24^\circ\text{C}$ under a $14\text{-hour}$ light/$10\text{-hour}$ dark cycle. After $6\text{ weeks}$, the dry biomass of the plants and the concentration of $Pb^{2+}$ in the plant tissues were measured.

To evaluate the specific influence of each experimental variable, the scientist must compare the results of a test group to a corresponding control or baseline group. Match each of the following experimental objectives with the group that serves as the most appropriate control or baseline.