Answer:
between-subjects design
Explanation:
According to my research on different experimental designs, I can say that based on the information provided within the question the best way to make sure that the internal validity will not be influenced by an outside event is to switch to a between-subjects design. This type of design is an experiment design that has two or more groups of subjects each being tested by a different testing factor simultaneously. Since they are done simultaneously it significantly reduces outside influence.
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It has to be d. The others wouldn't really make sense.
Smaller veins called venules then go back into the larger veins that pushes the blood back to the heart
