### Passage
To study the biodegradation of a synthetic polymer, polyethylene terephthalate (PET), by a newly isolated bacterium (*Thermobacillus petrolei*), researchers conducted two experiments.
In Experiment 1, five test tubes were prepared with of a liquid nutrient medium, of PET film, and of an active *T. petrolei* suspension. The pH and temperature were kept constant. A different concentration of a synthetic surfactant, Surfactant X, was added to Tubes 1–4. Tube 5 did not receive Surfactant X. After 7 days, the remaining mass of PET was measured to evaluate degradation.
In Experiment 2, to determine whether the PET degradation observed in Experiment 1 was due to active bacterial metabolic activity rather than non-biological chemical hydrolysis or passive physical absorption onto the bacterial biomass, the researchers prepared four additional test tubes (Tubes 6–9) under the same environmental conditions as Experiment 1, but modified the tube contents as shown below:
| Tube | Liquid Nutrient Medium | PET Film | *T. petrolei* Suspension | Surfactant X |
|---|---|---|---|---|
| 6 | Yes | Yes | No (sterile water added instead) | None |
| 7 | Yes | No | Yes (active cells) | None |
| 8 | No (sterile water added instead) | Yes | Yes (active cells) | None |
| 9 | Yes | Yes | Yes (heat-killed cells instead) | None |
To demonstrate that the degradation of PET film in Experiment 1 was specifically caused by the active metabolism of living *T. petrolei* cells rather than passive physical absorption of the polymer by the bacterial biomass, which of the following tubes from Experiment 2 serves as the most appropriate control when compared to Tube 5?
- ATube 6, because it contains no *T. petrolei* cells, allowing researchers to measure the rate of non-biological chemical hydrolysis of PET.
- BTube 7, because it contains no PET film, allowing researchers to measure the baseline cell density of *T. petrolei* in the liquid medium.
- CTube 8, because it contains no liquid nutrient medium, allowing researchers to determine if the bacteria can degrade PET in the absence of other nutrients.
- Tube 9, because it contains heat-killed *T. petrolei* cells, allowing researchers to isolate the effect of active bacterial metabolism from passive polymer absorption by the bacterial biomass.Answer