Necessary, because if it were not true — if the amount of trash on the city’s streets WAS declining at the same rate or faster before the campaign began than it did during the campaign — then we have no reason to think the campaign caused the reduction in trash. The reduction could have simply been a continuation of whatever causal forces were already in place prior to the campaign.

Not necessary, because the reasoning isn’t based on the people who believe that the campaign was not a success. Those people are mentioned purely as context — but they play no role in the reasons given for the conclusion.

Not necessary, because the argument does not compare the “clean City” campaign to any other campaign.

Not necessary, because whether the author of the argument has received funding to say what she says has no bearing on the logic of her argument.

Not necessary, because whether the trash declined steadily or decline ins some other pattern (like a sudden drop) is irrelevant. The campaign can still have a causal impact on reduction of trash without that reduction occurring at a steady rate over the course of the campaign. Maybe, for example, it takes time for the effect of a campaign to build, and the effects occur all at once toward the end of the campaign.