In 1907, a horse named clever hans was said to be able to answer math problems posed to him. he would tap out the correct answers with his hoof. however, an investigation revealed that if clever hans could not see the person asking the question, he did not provide correct answers. what does this suggest?
a : this illustrates the importance of including control groups -- without a second horse, no conclusion can be drawn.
b : clever hans was responding to subtle visual cues from the questioner.
c : clever hans was motivated to answer correctly by social interaction.
d : all of the choices are correct.

d. all of the choices are correct.

Explanation:

To be sure that this horse is a clever horse, need to be compared with other horses, that is the reason why is necessary a control group and contrast the results.

Also could be that the horse by visual contact, is a conductive behavior, for example the wink of a person. In this case the control group is also useful to compare results.

Horses like people, so another explanation is that by visual contact, the animal feel security and is easily to it to answer the problems.

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the correct answer is 2 and 4