Here is a question about this article: Historians estimate that between the advent of Islam in 650CE and the abolition of slavery in the Arabian Peninsula in the mid-20th century, 10 to 18 million sub-Saharan Black Africans were enslaved by Arab slave traders and transported to the Arabian Peninsula and neighboring countries. This number far exceeded the number of slaves who were taken to the Americas. Several factors affected the visibility of descendants of this diaspora in 21st-century Arab societies: The traders shipped more female slaves than males, as there was a demand for them to serve as concubines in harems in the Arabian Peninsula and neighboring countries. Male slaves were castrated in order to serve as harem guards. The death toll of Black African slaves from forced labor was high. The mixed-race children of female slaves and Arab owners were assimilated into the Arab owners' families under the patrilineal kinship system. As a result, few distinctive Afro-Arab black communities have survived in the Arabian Peninsula and neighboring countries.
What is the answer to this question: Why were women slaves more popular?
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So... for them to serve as concubines

Here is a question about this article: On the Origin of Species, published on 24 November 1859, is a work of scientific literature by Charles Darwin which is considered to be the foundation of evolutionary biology. Darwin's book introduced the scientific theory that populations evolve over the course of generations through a process of natural selection. It presented a body of evidence that the diversity of life arose by common descent through a branching pattern of evolution. Darwin included evidence that he had gathered on the Beagle expedition in the 1830s and his subsequent findings from research, correspondence, and experimentation.
What is the answer to this question: How do populations evolve according to Charles Darwin's theory?
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So... through a process of natural selection

Here is a question about this article: Television money had also become much more important; the Football League received 6.3 million for a two-year agreement in 1986, but when that deal was renewed in 1988, the price rose to 44 million over four years. The 1988 negotiations were the first signs of a breakaway league; ten clubs threatened to leave and form a "super league", but were eventually persuaded to stay. As stadiums improved and match attendance and revenues rose, the country's top teams again considered leaving the Football League in order to capitalise on the growing influx of money being pumped into the sport.
What is the answer to this question: For how many years was the 1986 television rights contract?
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So...
two