This is a version of our prediction, and it is necessary. We identified the need for bacteria to be present, and (A) is stating that some landfills fulfill this requirement. If no landfills have bacteria, then there is no threat of vapor, and no damage to human health.

This is a conditional that is not necessary. It wouldn’t be a correct sufficient assumption answer either, because it’s switching necessary and sufficient.

The negation of this is “If a practice involves... then it doesn’t necessarily cause some damage to human health.” This means that it still could cause damage, so the argument is not ruined. Be careful not to go too far in negation.

Irrelevant. The argument concludes that the practice is harmful. It isn’t necessary that we have something to combat the harm.

Too strong. We don’t need a conditional rule that speaks for all landfills. We only need for this to be true in one instance in order for the conclusion (that there is damage) to stand a chance.