This does not weaken the argument, because it doesn’t challenge the author’s cost-benefit assumption. This gives us no reason to believe that oil heating now represents a cost savings over gas heating—we still don’t know which one is currently cheaper.

This does not weaken the argument because it doesn’t indicate any kind of savings from switching back to oil. We’re only concerned with homeowners who currently use gas, so the cost of switching to gas is irrelevant. This also doesn’t tell us how oil and gas prices compare.

This does not weaken the argument. We’re not concerned with gas heating equipment prices, because we only care about current gas users. Like (E), the falling cost of oil is tempting, but we still don’t know how it compares with gas—maybe they both fell, and gas is still cheaper.

This weakens the argument because it indicates that oil is currently less expensive than gas, while also reducing the additional cost of getting new oil-heating equipment. This undermines the author’s cost-benefit assumption, thereby weakening.

This does not weaken the argument, because like (C), just knowing that the cost of oil has fallen doesn’t actually tell us much: we still don’t know how oil and gas compare. The continued decline in oil heating also doesn’t do anything, because we don’t know why it’s happening.