Input: Read this: According to a 2011 Pew Research Center survey, there were 2.2 billion Christians around the world in 2010, up from about 600 million in 1910. By 2050, the Christian population is expected to exceed 3 billion. According to a 2012 Pew Research Center survey Christianity will remain the world's largest religion in 2050, if current trends continue.
Question: Which survey conducted in 2012, concluded there were about 2.2 billion Christians worldwide?

Output: unanswerable

Input: Read this: By 500 BC, the Persian Empire controlled the Greek city states in Asia Minor and had made territorial gains in the Balkans and Eastern Europe proper as well. Attempts by some of the Greek city-states of Asia Minor to overthrow Persian rule failed, and Persia invaded the states of mainland Greece in 492 BC, but was forced to withdraw after a defeat at the Battle of Marathon in 490 BC. A second invasion by the Persians followed in 480 BC. Despite a heroic resistance at Thermopylae by Spartans and other Greeks led by King Leonidas, and a simultaneous naval engagement at Artemisium,[page needed] Persian forces occupied Athens, which had been evacuated in time, as well as briefly overrunning half of Greece. Following decisive Greek victories in 480 and 479 BC at Salamis, Plataea, and Mycale, the Persians were forced to withdraw for a second time, marking their eventual withdrawal from all of their European territories. Led by Athens and Sparta, the Greek victories in the Greco-Persian Wars are considered a pivotal moment in world history, as the 50 years of peace that followed are known as Golden Age of Athens, the seminal period of ancient Greece that laid many of the foundations of Western civilization.
Question: What battle did Persia lose in 490 BC?

Output: Battle of Marathon

Input: Read this: Most of Hume's followers have disagreed with his conclusion that belief in an external world is rationally unjustifiable, contending that Hume's own principles implicitly contained the rational justification for such a belief, that is, beyond being content to let the issue rest on human instinct, custom and habit. According to an extreme empiricist theory known as phenomenalism, anticipated by the arguments of both Hume and George Berkeley, a physical object is a kind of construction out of our experiences. Phenomenalism is the view that physical objects, properties, events (whatever is physical) are reducible to mental objects, properties, events. Ultimately, only mental objects, properties, events, exist hence the closely related term subjective idealism. By the phenomenalistic line of thinking, to have a visual experience of a real physical thing is to have an experience of a certain kind of group of experiences. This type of set of experiences possesses a constancy and coherence that is lacking in the set of experiences of which hallucinations, for example, are a part. As John Stuart Mill put it in the mid-19th century, matter is the "permanent possibility of sensation". Mill's empiricism went a significant step beyond Hume in still another respect: in maintaining that induction is necessary for all meaningful knowledge including mathematics. As summarized by D.W. Hamlin:
Question: What is phenomenalism?

Output: physical objects, properties, events (whatever is physical) are reducible to mental objects, properties, events

Input: Read this: The Aksumites erected a number of large stelae, which served a religious purpose in pre-Christian times. One of these granite columns, the obelisk of Aksum, is the largest such structure in the world, standing at 90 feet. Under Ezana (fl. 320360), Aksum later adopted Christianity. In the 7th century, early Muslims from Mecca also sought refuge from Quraysh persecution by travelling to the kingdom, a journey known in Islamic history as the First Hijra. It is also the alleged resting place of the Ark of the Covenant and the purported home of the Queen of Sheba.
Question: Which stelae is considered the largest in the world, standing at 90 feet?

Output:
the obelisk of Aksum