Robert Completed His PhD Thesis on the Different Attitudes of Indian

Robert completed his PhD thesis on the different attitudes of Indian migrants and Sri Lankan refugees towards immigration officials. This was carried out using a standard interview conducted in the preferred language of each participant. His examiners were critical of his work, because they were not sure that his samples of the two groups were equivalent. Which of these is most likely to have been the reason for that criticism?

A) Indian and Sri Lankan people are not necessarily from different cultures.
B) One group were refugees, the other voluntary migrants, and their experiences of immigration might be different.
C) The research context was informal.
D) Participants may not have been experienced in interview situations.