Evolutionary Biology - why does Nucleotide Diversity differs between autosomes and X- chromosomes ?

In many species the nucleotide diversity differs between the autosomes and X-chromosome. For example, in humans the average neutral nucleotide diversitiies are: autosomes = 1 x 10-3 per site, X-chromosome = 0.6 x 10-3 per site.

a. How do you explain the difference in the level of diversity of the autosome and X-chromosome? (Hint: there are at least two explanations). (4 marks)

b. How would you determine which of your explanations was correct (Note: evolution typically proceeds too slowly to start an experiment) (2 marks)

c. In contrast in African D. melanogaster the relative neutral diversities are: autosomes = 1.8 x 10-2 and X-chromosome = 1.8 x 10-2 per site. How might you explain this pattern given your explanation above? (Hint: there are at least two explanations). (4 marks)