What is the primary difference between an experiment and an observational study?
A. An observational study involves many more people.
B. Observational studies always make correlations.
C. In an experiment, the researcher manipulates a variable.
D. Experiments are always done in a lab.

Read the passage. excerpt from "why equal pay is worth fighting for" by senator elizabeth warren, april 17, 2014 i honestly can't believe that we're still arguing over equal pay in 2014. when i started teaching elementary school after college, the public school district didn't hide the fact that it had two pay scales: one for men and one for women. women have made incredible strides since then. but 40 years later, we're still debating equal pay for equal work. women today still earn only 77 cents for every dollar a man earns, and they're taking a hit in nearly every occupation. bloomberg analyzed census data and found that median earnings for women were lower than those for men in 264 of 265 major occupation categories. in 99.6 percent of occupations, men get paid more than women. that's not an accident; that's discrimination. the effects of this discrimination are real, and they are long lasting. today, more young women go to college than men, but unequal pay makes it harder for them to pay back student loans. pay inequality also means a tougher retirement for women. . for middle-class families today, it usually takes two incomes to get by, and many families depend as much on mom's salary as they do on dad's, if not more. women are the main breadwinners, or joint breadwinners, in two-thirds of the families across the country, and pay discrimination makes it that much harder for these families to stay afloat. women are ready to fight back against pay discrimination, but it's not easy. today, a woman can get fired for asking the guy across the hall how much money he makes. here in the senate, sen. barbara mikulski (d-md.) introduced the paycheck fairness act to give women the tools to combat wage discrimination. it would ensure that salary differences have something to do with the actual job that they are doing, and not just because they are women. senator warren states that the effects of pay discrimination are long-lasting. is this a valid argument supported by accurate evidence? no; warren weakens her point by claiming that the paycheck fairness act would "give women the tools to combat wage discrimination." yes; warren supports her point by noting, "for middle-class families today, it usually takes two incomes to get by." yes; warren supports her point by noting, "pay inequality also means a tougher retirement for women." no; warren weakens her point by noting, "today, a woman can get fired for asking the guy across the hall how much money he makes."