Question

troubled waters
the northwest passage is a sea route from the atlantic ocean westward to the pacific. located north of the arctic circle, the route is filled with thousands of huge icebergs. explorers began searching for the passage in the late 1400s. they wanted to find a shorter route from europe to asia. over many years, explorers such as francis drake and henry hudson tried to find the passage. all had problems sailing in the icy waters. the norwegian explorer roald amundsen finally completed the difficult voyage by sea in 1905. it took amundsen and his crew three years to make the dangerous journey to alaska.
what is the main, or central, idea of the passage?
though it took him three years, explorer roald amundsen successfully sailed the entire northwest passage in 1905.
the northwest passage is filled with large icebergs and is a dangerous route to travel.
because the northwest passage is so difficult to sail through, explorers took hundreds of years to find it.

Explanation:

The passage first introduces the Northwest Passage, then mentions explorers' long - term search (from the late 1400s) due to its difficulty (icy waters, icebergs). Finally, it highlights Amundsen's success in 1905 after years of exploration. The key is the long - term effort (hundreds of years) by explorers to find the passage because of its sailing difficulty.

Answer:

Because the Northwest Passage is so difficult to sail through, explorers took hundreds of years to find it.