GMAT Sentence Correction (SC) | As the housing affordability gap widens, middle-income

I am yet to understand why C is makes more sense than B. that's why I choose B.
C introducing contrast which is not required in the sentence. As the underlined statement is just a extension of the first sentence. Hence, FURTHERMORE will be more appropriate than YET.
When I go deep into the meaning. I feel SINCE is making the statement incorrect. Because of may be C is the right choice..

Please clarify.