All of the power doesn't go to one party. There is an option for two. And it forces people to decide what they think to do with only two parties instead of more than two
The author's historical context can be relevant. For example, a passage from the bible will have different biases than that of an Islamic scholar. This highly affects what their words will mean and what they'll omit/include. Both of these allow you to form a fully sourced claim and rebuttals in your essay or short response.
The correct answer is B., to show congress and the president the strength of the civil rights movement.