Lets understand the experimental setup by breaking down the method.
A group of (I) middle school students were given a short course in speed-reading. The instructor was curious if a (II) monetary incentive would influence performance on a reading test taken at the end of the course. Half the students were offered $5 for obtaining a certain level of performance on the test, the (III) other half were not offered money.
(I) The number of middle school students present will be the constant of the experiment as it is not changing.
(II) Monetary incentive would influence performance on a reading test. This statement reflects that monetary incentive of $5 is the independent variable and the performance on a reading test is the dependent variable.
(III) The other half were not offered money. This statement mentions the control group of the experiment who are not receiving money regardless of their performance in the test. The comparison of the case with incentive of $5 to the control group will help establish any causal effect.
Based on the above understanding of the passage, here's the answer,
1 - (d), 2 - (a), 3 - (b), 4 - (c)
