TOEFL Listening: TPO-TOEFL阅读TPO - X37FT6ZZLT008525N$

According to paragraph 1, allopatric speciation is possible when A. a population contains all the different genes present in a species at a particular time B. a population becomes isolated due to the presence of a geographic barrier C. genetic mixing begins to occur in previously separate populations of a species D. a species is successful in crossing a geographic barrier