Which of the Following Is an Example of a Difference

Which of the following is an example of a difference in two proportions based on independent samples?

A) A random sample of 1000 voters is asked who they plan to vote for in the upcoming election. The difference is found between the proportion of voters who plan to vote for the Republican candidate and the proportion of voters who plan to vote for the Democratic candidate.
B) Each student in a random sample of 500 sophomores and a random sample of 500 seniors is asked what proportion of classes he or she skips in a typical quarter. The difference in the average responses for the two groups is found.
C) A random sample of 800 adults in the US in 1999 was asked if they support the legalization of marijuana, and another random sample of 800 adults was asked the same question is 2009. The difference in the proportions of adults who supported it in 1999 and 2009 is found.
D) None of the above is an example of a difference in two proportions based on independent samples.