Psychologists have conducted more research on language comprehension than on language production. Which of the following students provides the best explanation for this discrepancy?

a. Tina: "Psychologists find language comprehension to be more interesting from the perspective of social interactions."
b. Nayan: "Researchers find it easier to conduct research on language comprehension, because they can easily vary the material that participants hear or see."
c. Garth: "Researchers in the discipline of speech therapy have already conducted the relevant research on language production."
d. Louise: "Language production was the major topic of research just after the cognitive approach became popular; in the last decade, language comprehension has become the major topic of research."