What is the central idea of the passage? (passage is an excerpt from The Unicorn of the Sea by the National Oceanic and Atmosphere Administration) a. whales are difficult to study because they are found in waters that are difficult to reach and covered in ice.
b. the narwhal has a single tooth, outside of its mouth, unlike other marine mammals, who have many teeth.
c. whales, such as the narwhal, that inhabit Arctic waters feed mainly during the spring and summer months.
d. the narwhal is a deep-water diving marine mammal with a single tooth, or tusk, for which it is best known.