Detailed Instructions: You need to read the given passage and construct a question about the information present in the passage. The question should be answerable from the passage. Most importantly, only those questions are allowed whose answers cannot be determined from a single sentence. Do not generate questions that require only a single sentence to answer. Instead, try to combine two sentences and create a question from that combination. Do not create open-ended questions, i.e., the answers to them cannot be found precisely from the information provided in the passage.
Q: Paragraph- Sent 1: The inhabited history of the Las Vegas Valley stretches to 23,000 b.c.
Sent 2: , when much of the area was covered by a prehistoric lake.
Sent 3: During this period, the indigenous people lived in caves, hunting the mammals that gathered at the shoreline.
Sent 4: The landscape of the valley changed dramatically over the next 200 centuries.
Sent 5: The glaciers feeding the lake melted away and the lake evaporated.
Sent 6: Fossils tell an obscure story of man's slow and sporadic development.
Sent 7: Around 3000 b.c.
Sent 8: , native Archaic Indians began to develop a lasting hunting and gathering culture.
Sent 9: By this time, the valley was in much the same geographic state as it exists in today, with one exception — the presence of artesian springs that bubbled to the surface in several areas.
Sent 10: These springs fed a network of streams draining through the Las Vegas Wash to the Colorado River.
Sent 11: The areas surrounding the springs were desert oases: sprawling collections of grasses, trees, and wildlife.
Sent 12: Many springs lay in areas that would eventually become the center of the modern Las Vegas metropolis.
Sent 13: For about 4000 years, the Archaics thrived in a culture that included many signs of early civilization.
Sent 14: Signs of even more advancement appeared halfway through the first millennium a.d.
Sent 15: , when the Anasazi Indians inhabited the valley.
Sent 16: Far more progressive than the Archaics, the Anasazi utilized such formal agricultural techniques as irrigation to assist their harvest.
Sent 17: This permitted the Anasazi to achieve a benchmark of advanced society — the ability to live in permanent shelters year-round without need to follow wildlife.
Sent 18: Mysteriously, the Anasazi vanished from the valley around a.d.
A:
Why did the prehistoric lake disappear from the Las Vegas Valley?