What are the possible confounding variables in this study?

The single most powerful person in the entire Congress is the ____________________

Fill in the blank(s) with correct word

During the nomination campaign, __________, while during the general election campaign __________

A) parties in each state hold a primary or caucus; the electoral college casts its ballots B) parties in each state hold a primary or caucus; we are likely to see multiple televised debates between the candidates of the two major parties C) parties in each state hold a primary or caucus; each party holds a national convention to showcase its candidate D) potential candidates explore the possibility of running for president; each party holds a convention to showcase its candidate E) potential candidates explore the possibility of running for president; we are likely to see multiple televised debates between the candidates of the two major parties