Machu Picchu is an extraordinary place for many reasons. It's an ancient city on a 9,000-foot mountain top with stunning views, but beyond its obvious beauty, it's fascinating because it cannot be seen from below. Historians now think this city from the early 1400s was a secret place. It was home to more than 150 houses and palaces, temples and baths—all up in the clouds. The buildings were carved out of the granite from the mountain. No one knows for sure what the Inca did there, but it was almost certainly used as an astronomic observatory. The Spanish never found Machu Picchu.

An advantage of a building that cannot be seen from below is

A
it can have a lot of buildings.

B
it can serve as a welcome center to visitors.

C
it can remain hidden from invaders.

D
it is easily found by people looking for it.