I would say that a) decrease in gas is positive (unless you sell the gas: then it is a potential stressor)
b) this would be a positive surprise, not a stressor
c) this would also be a positive surprise, not a stressor
d) I think that this would be a potential stressor: especially if you don't feel comfortable with the people who visit, or would have preferred to clean up the flat, or don't have time for the visit: this would be my answer.
