Once aboard, the explorers set sail and used maps to navigate through known territory. The maps were not useful when sailors could not see land or when they were in unknown territory. How would they navigate across the vast Atlantic Ocean without any land markers to guide them? This problem led to the invention of navigation equipment. Compasses have a magnetized metal needle that always points north. Sailors use them as a point of reference to tell if their ship is in the right position to move forward. In order to find their location, sailors used astrolabes, sextants, cross staffs, or quadrants to find latitude. With these tools, explorers were able to sail on to discover new lands.

Which statement from the text supports the main idea that sailors had difficulty navigating uncharted waters?

a
Compasses have a magnetized metal needle that always points north.
b
In order to find their location, sailors used astrolabes, sextants, cross staffs, or quadrants to find latitude.
c
Sailors use them as a point of reference to tell if their ship is in the right position to move forward.
d
The maps were not useful when sailors could not see land or when they were in unknown territory.