Answer:
another woman reading the same poem
Explanation: For a research to be appropriately conducted, it must be valid and reliable, Scientists include a control or more than one conditions for analysis to make sure the results of their experimental research can be compared between the main measurements and the control measures or other conditions in order to increase the reliability of the outcomes and make their research valid.
Here to make the research valid, a first condition which involved the infants hearing their mother;s voice and the second condition which , infants heard another woman voice reading the same poem are being conducted to draw reasonable conclusions whether infants can identify their mother through voice.