The microbiologist is only addressing those bacteria that are resistant to both heavy-metal poisoning and antibiotics. Even if (A) is true, it wouldn’t impact his conclusion that heavy-metal exposure causes antibiotic resistance in certain bacteria.

This strengthens the hypothesis by providing a control group. If bacteria in sludge without heavy metals aren’t resistant to heavy-metal poisoning or antibiotics, it’s more likely that the heavy-metal exposure did cause antibiotic resistance in the bacteria in the other sludge.

This weakens the microbiologist’s hypothesis by suggesting that the causal relationship is reversed. That is, (C) suggests that antibiotic resistance causes heavy-metal poisoning resistance, rather than the other way around.

This weakens the hypothesis by providing a plausible alternative explanation. If sludge that contains heavy metals also contains significant amounts of antibiotics, it’s likely that the antibiotics cause antibiotic resistance in the bacteria, not the heavy-metal exposure.

This doesn’t strengthen the hypothesis that heavy-metal exposure caused antibiotic resistance in the bacteria. We don’t know if the bacteria described in (E) were exposed to heavy metals or not.