The most logical reason for using a group of naked mole rats that had been in one room for many generations in Elizabeth Clare's study is that the DNA would have built up in the air during the time they had been there. This would allow researchers to analyze any accumulated genetic material from the population in a controlled environment, facilitating their study on environmental DNA (eDNA). Thus, the correct response is:
The DNA would have built up in the air during the time they had been there.