Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 31 to 35.

The Great Pyramid of Giza was built as a tomb for Pharaoh Cheops in 2720 B.C. Certain aspects of its construction make it one of the true wonders of the world. The thirteen-acre structure near the River Nile is a solid mass of stone blocks covered with limestone. Inside are a number of hidden passageways and the burial chamber of the Pharaoh. It is the largest single structure in the world and is an incredible engineering feat. The ancient Egyptians were sun worshippers and great astronomers, so calculations for the Great Pyramid were based on astronomical observations.

Explorations and detailed examinations of the base of the structure reveal many intersecting lines. Further scientific study indicates that these represent a type of timeline of events - past, present and future. Many of the events have been interpreted and found to coincide with known facts of the past. Others are prophesied for future generations and are currently under investigation. Many believe that pyramids have supernatural powers and this one is no exception. Some researchers even associate it with extraterrestrial beings of ancient past.

On what did the ancient Egyptians base their calculations?