Making a choice stage, let me know if I'm wrong.
Look over each of the choices, and plug in each answer to see which one makes more sense, or rather, adds understanding to what you are reading. Start from the bottom. Adding 'and' does little in the way of allowing the reader to understand better. The same can be said for explaining what a tax burden is. By stating the 'therefore', you are saying that what comes after is a factual statement. Adding 'however' is the only answer that improves the relationship between the two ideas.
