The great pyramids at giza are remarkable structures because a. they are still the most massive structures ever built by human effort
b. engineers have not figured out how they were built without modern technology
c.. they were the tallest man-made structures until the 20th century
d. they were the most massive man-made structures until the 20th century

The best and most correct answer among the choices provided by the question is letter B "engineers have not figured out how they were built without modern technology "

The Great Pyramid of Giza (also known as the Pyramid of Khufu or the Pyramid of Cheops) is the oldest and largest of the three pyramids in the Giza pyramid complex bordering what is now El Giza, Egypt. It is the oldest of the Seven Wonders of the Ancient World, and the only one to remain largely intact.

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