A psychologist wants to know whether students' mid-term exam scores were impacted by COVID-19. She looks at the mid-term exam scores from 2019 and last year's mid-term exam scores to compare. She categorizes the two study conditions as "Before COVID-19" and "After COVID-19", and she expect that COVID-19 will impact students' mid-term exam scores. What type of variable is the independent variable? a. Quantitative variable b. Discrete variable c. Qualitative variable d. Continuous variable