Please explain

The conclusion of the argument is: "tax reform is in the interest of low-income taxpayers." Why? Because we are told that the annual tax burden on low-income taxpayers will be reduced, on average, by anywhere from $100 to $300. We are looking or the answer choice that will weaken the conclusion. Answer choice (B) states: "Tax reform, by eliminating tax incentives to build rental housing, will push up rents an average of about $40 per month for low-income taxpayers." So, even though the tax reform is saving low-income taxpayers between $100-$300 a year, the tax reform is pushing up rents for low-income taxpayers an average of $480 ($40 x 12). Thus, it no longer stands that the tax reform is in the interest of low-income taxpayers. Hope that clears things up! Please let us know if you have any other questions.

how do we know that it is 100-300 a year?