Based on the information present in the given passage, you need to write a correct answer to the given question. A correct answer is one that correctly and completely answers the question. Do not write answers that address the question partially or incorrectly. The correct answer must be contained in the given paragraph.
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Question: Paragraph- Sent 1: Fossils can be used to match up rock layers.
Sent 2: As organisms change over time, they look different.
Sent 3: Older fossils will look different than younger fossils.
Sent 4: Some organisms only survived for a short time before going extinct.
Sent 5: Knowing what organisms looked like at certain times also helps date rock layers.
Sent 6: Some fossils are better than others for this use.
Sent 7: The fossils that are very distinct at certain times of Earths history are called index fossils.
Sent 8: Index fossils are commonly used to match rock layers.
Sent 9: You can see how this works in Figure 2.30.
Sent 10: If two rock layers have the same index fossils, then they're probably about the same age.
Question: What helps to date rock layers?.

Answer: Index fossils.

Question: Paragraph- Sent 1: Alexander fled Macedon with his mother, dropping her off with her brother, King Alexander I of Epirus in Dodona, capital of the Molossians.
Sent 2: He continued to Illyria, where he sought refuge with the Illyrian King and was treated as a guest, despite having defeated them in battle a few years before.
Sent 3: However, it appears Philip never intended to disown his politically and militarily trained son.
Sent 4: Accordingly, Alexander returned to Macedon after six months due to the efforts of a family friend, Demaratus, who mediated between the two parties.
Sent 5: In the following year, the Persian satrap (governor) of Caria, Pixodarus, offered his eldest daughter to Alexander's half-brother, Philip Arrhidaeus.
Sent 6: Olympias and several of Alexander's friends suggested this showed Philip intended to make Arrhidaeus his heir.
Sent 7: Alexander reacted by sending an actor, Thessalus of Corinth, to tell Pixodarus that he should not offer his daughter's hand to an illegitimate son, but instead to Alexander.
Sent 8: When Philip heard of this, he stopped the negotiations and scolded Alexander for wishing to marry the daughter of a Carian, explaining that he wanted a better bride for him.
Sent 9: Philip exiled four of Alexander's friends, Harpalus, Nearchus, Ptolemy and Erigyius, and had the Corinthians bring Thessalus to him in chains.
Question: Why did Olympias and several other friends of Alexander, think that Philip intended to make Arrhidaeus his heir?.

Answer: Because Persian satrap (governor) of Caria, Pixodarus, offered his eldest daughter to Alexander's half-brother, Philip Arrhidaeus.

Question: Paragraph- Sent 1: Tuesday, September 11, 2001, dawned temperate and nearly cloudless in the eastern United States.
Sent 2: Millions of men and women readied themselves for work.
Sent 3: Some made their way to the Twin Towers, the signature structures of the World Trade Center complex in New York City.
Sent 4: Others went to Arlington, Virginia, to the Pentagon.
Sent 5: Across the Potomac River, the United States Congress was back in session.
Sent 6: At the other end of Pennsylvania Avenue, people began to line up for a White House tour.
Sent 7: In Sarasota, Florida, President George W. Bush went for an early morning run.
Sent 8: For those heading to an airport, weather conditions could not have been better for a safe and pleasant journey.
Sent 9: Among the travelers were Mohamed Atta and Abdul Aziz al Omari, who arrived at the airport in Portland, Maine.
Sent 10: Boston: American 11 and United 175.
Sent 11: Atta and Omari boarded a 6:00 A.M. flight from Portland to Boston's Logan International Airport.
Sent 12: When he checked in for his flight to Boston, Atta was selected by a computerized prescreening system known as CAPPS (Computer Assisted Passenger Prescreening System), created to identify passengers who should be subject to special security measures.
Sent 13: Under security rules in place at the time, the only consequence of Atta's selection by CAPPS was that his checked bags were held off the plane until it was confirmed that he had boarded the aircraft.
Sent 14: This did not hinder Atta's plans.
Sent 15: Atta and Omari arrived in Boston at 6:45.
Sent 16: Seven minutes later, Atta apparently took a call from Marwan al Shehhi, a longtime colleague who was at another terminal at Logan Airport.
Sent 17: They spoke for three minutes.
Sent 18: It would be their final conversation.
Question: How long was Atta's final conversation with Marwan al Shehhi?.

Answer:
Three minutes.