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.

[EX Q]: 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: How can extinction help to date rock layers?.
[EX A]: Some animals only existed during certain time periods before becoming extinct. It helps to date that layer of rock where these animal fossils are present.

[EX Q]: Paragraph- Sent 1: Patricia Cross and her boyfriend Larry Osborne , two students in a San Francisco school , become expelled for the publication of an off-campus underground paper .
Sent 2: As a result , a philosophy professor , Dr. Jonathon Barnett , resigns his teaching position and decides to become an advocate for the counterculture youth movement and , specifically , the use of LSD .
Sent 3: The hippies of the Haight-Ashbury district first see him as a hero and then as something even more .
Sent 4: Dr. Barnett even makes an appearance on the Joe Pyne TV show to voice his support of the hippie community and the use of LSD .
Sent 5: One scheming young man sees the opportunity to build Dr. Barnett as the head of a cult centered around the use of LSD .
Sent 6: He hopes to earn profit from the users , Dr. Barnett's speeches known as '' happenings , '' and their lifestyles .
Sent 7: At a massive LSD-fueled dance , Patricia begins to have a bad trip Which leads to an argument between her and Pat , ultimately splitting the couple up .
Sent 8: After Patricia realizes that she's pregnant , Dr. Barnett advises her to have an abortion , ultimately leading to Patricia attempting suicide .
Sent 9: However , Larry saves her and makes the destruction of Dr. Barnett's cult his primary objective .
Sent 10: Larry shoots Dr. Barnett from the crowd at one of his massive speeches .
Sent 11: As another hippie in attendance calms the audience and Elliot sees his new leader for their cult-like organization , Larry realizes that his assassination of Dr. Barnett simply made him a martyr for the hippie movement .
Question: What made Dr. Jonathon Barnett to resign his teaching position?.
[EX A]: To become advocate for the counterculture youth movement.

[EX Q]: Paragraph- Sent 1: There once was a guy named Kevin and girl named Erin.
Sent 2: They really liked spending time together.
Sent 3: One day, Kevin traveled from his home in the United States of America to Erin's house in England.
Sent 4: They then thought to take a sudden trip around the world.
Sent 5: They first traveled by plane around Europe, where they saw many different people and sights.
Sent 6: They then took a boat to Africa and Asia, where they went on a trip through the mountains.
Sent 7: Later in the month, they traveled to China by train and were allowed to see how different life was over there.
Sent 8: Next they took another plane to Australia, where they had a lot of fun seeing kangaroos and a different type of English speaking people.
Sent 9: After spending a week in Australia, Kevin and Erin took a really long plane ride to North America, where they drove across the land.
Sent 10: They saw everything from the mountains to forests.
Sent 11: They even got to visit the beach!
Sent 12: Because they had so much fun, Kevin returned home with Erin to England where they hung out and spent the next few days and months talking about all of the neat things they saw and did on their trip.
Question: Kevin and Erin went to what country after they took a train?.
[EX A]:
Australia.