Refer to the Information Provided in Scenario 21

Refer to the information provided in Scenario 21.3 below to answer the questions that follow.
SCENARIO 21.3: Consider a local high school that has 300 sophomores considered to be at-risk students. A privately-sponsored after-school sports program has been established to try to help at-risk students succeed in finishing high school. The local government wants to know if this program reduces the students' probability of dropping out of high school before the end of their junior year, and decides to randomly sample 150 of the 300 at-risk sophomores and invite them to attend the after-school program at no cost. 100 students accept the invitation, and by the end of their junior year, the drop-out percentage of the 100 students who attended the program was lower than the drop-out percentage of the 200 students who did not attend the program.
-Refer to Scenario 21.3. Based on the information in the scenario, the 100 students who accepted the offer to attend the after-school program is an example of

A) selection bias.
B) a random sample.
C) survivor bias.
D) positive correlation.