What Was the Relevance to Modern-Day American Politics of a Political

What was the relevance to modern-day American politics of a political science professor's finding that congressional party unity in legislative voting has regularly exceeded 90 percent since the 1980s, while hovering around 20 percentage points lower over the previous three decades?

A) The research implied that party leaders of earlier decades were ineffective.
B) The parties, the professor showed, became more ideologically distinct or homogenous after 1980, with fewer moderates in their ranks.
C) The two parties began digging in their heels, the research implied, as more people sought alternatives to traditional Democratic and Republican formulas.
D) The parties had successfully carried off long-term, leadership- and marketing-driven efforts to give voters clear choices when considering party brands.