Which One of the Following Ways of Collecting Data Would

Which one of the following ways of collecting data would result in two independent samples?

A) A sample of college students is measured for stress both before and after taking an important exam.
B) A group of college students are paired based on gender, age, major, and grades. One student in a pair receives stress management training and the other one does not. After a few weeks all students are measured for stress right before an important exam.
C) A sample of English majors is measured for stress before taking an important exam. A sample of engineering majors is also measured for stress before taking their important exam.
D) A sample of college students is measured for stress before taking an important exam. Their exam grade is also recorded.