A、They are spread randomly across the world. B、Some are more difficult to learn than others. C、More are found in tropical region

Last week, I attended a research workshop on an island in the South Pacific. Thirty people were present, and all except me came from the island, called Maklua, in the nation of Vanuatu. They live in 16 different communities and speak 16 distinct languages.
    In many cases, you could stand at the edge of one village and see the outskirts of the next community. [19] Yet the residents of each village speak a completely different language. According to recent work by my colleagues at the Max Plank Institute for the Science of Human History, this island, just 100 kilometers long and 20 kilometers wide, is home to speakers of perhaps 40 different indigenous languages. Why so many?
    We could ask the same question of the entire globe. People don’ t speak one universal language, or even a handful. [20] Instead, today our species collectively speaks over 7,000 distinct languages. And these languages are not spread randomly across the planet. For example, far more languages are found in tropical regions than in the mild zones. The tropical island of New Guinea is home to over 900 languages. Russia, twenty times larger, has 105 indigenous languages.
    Even within the tropics, language diversity varies widely. For example, the 250,000 people who live on Vanuatu’s 80 islands speak 110 different languages. But in Bangladesh, a population 600 times greater speaks only 41 languages.
    How come humans speak so many languages? And why are they so unevenly spread across the planet? As it turns out, we have few clear answers to these fundamental questions about how humanity communicates.
    Most people can easily brainstorm possible answers to these intriguing questions. They hypothesized that language diversity must be about history, cultural differences, mountains or oceans dividing populations.
    But when our diverse team of researchers from six different disciplines and eight different countries began to review what was known, we were shocked that only a dozen previous studies had been done:, including one we ourselves completed on language diversity in the Pacific.
     [21] These prior efforts all examine the degree to which different environmental, social, and geographic variables correlated with a number of languages found in a given location. The results varied a lot from one study to another, and no clear patterns emerged. The studies also ran up against many methodological challenges, the biggest of which centered on the old statistical saying "correlation does not equal causation".
Questions 19 to 21 are based on the recording you have just heard.
19. What does the speaker say about the island of Maklua?
20. What do we learn from the talk about languages in the world?
21. What have the diverse team of researchers found about the previous studies on language diversity?