Final answer:
Lurking variables are a primary threat to internal validity in between-subjects experiments. Random assignment of subjects to treatment groups helps mitigate this threat by spreading potential lurking variables equally among the groups.
Step-by-step explanation:
One of the primary threats to internal validity in a between-subjects experiment is the presence of lurking variables. Lurking variables are additional variables that can influence the response variable and cloud the study's findings. To mitigate this threat, researchers must design experiments with random assignment of subjects to treatment groups, ensuring the potential lurking variables are spread equally among the groups. This way, any differences in the response variable can be attributed to the different treatments and not other factors.