Use the Following to Answer Questions

Use the following to answer questions
Scenario II
Scenario II is based on fabricated data inspired by the following study:
Black, J. E., Isaacs, K. R., Anderson, B. J., Alcantara, A. A. & Greenough, W. T. (1990) . Learning causes synaptogenesis, whereas motor activity causes angiogenesis, in cerebellar cortex of adult rats. Proceedings of the National Academy of Science, 87, 5568-5572.
Experience and Brain Plasticity Study
The purpose of this study was to examine how various types of motor experiences affect neuronal and vascular changes in the brain. To that end, rats were randomly assigned to one of four behavioral conditions. Rats in the inactive control (IC) condition were raised in standard rat cages with minimal access to physical activity. Rats in the voluntary exercise (VX) condition were raised in standard cages with free access to physical activity via an attached wheel. Rats in the forced exercise (FX) condition also participated in exercise, but were forced to do so on a motorized treadmill. Lastly, rats in the acrobatic condition (AC) engaged in physical activity that included learning a skill. Specifically, the AC rats were trained to traverse a series of tightropes that were progressively more difficult. After 30 days in their respective treatments all of the rats were sacrificed and their brains removed. The researchers examined the brains for synapse density and vascular density. ANOVA revealed animals in the AC condition had a significantly greater synapse density than the other treatment conditions. The results also revealed that the VX rats had a significantly greater blood vessel density compared with the IC rats, but not with the FX and AC rats. These results indicate different behavioral experiences have different effects on the brain's morphology.
-(Scenario II) Suppose the vendor who sold the rats to the researchers in Scenario II only had 20 rats left. The researchers needed a total of 40 rats to conduct this study so they buy the remaining 20 from another vendor. The strain, age, and sex of rats from both vendors were identical. Which of the following is a potential problem for this study?

A) The study is confounded and differences between the sample cannot be attributed to the independent variable.
B) The small sample size makes it difficult to establish internal validity.
C) The study is confounded because vendor type is an uncontrolled variable.
D) None of the above, because rats were randomly assigned to the treatment conditions.