4. Which excerpt would be most important to include in a summary of the article?
a. Before modern trade routes, salt was scarce in West Africa. People living there had to
obtain their salt from traders in North Africa.
b. In fact, the word "salary" derives from an ancient Roman practice.
c. It has just one electron on its valence shell, which can easily transfer.
d. Among the neurotoxins that work like this is tetrodotoxin. This toxin is produced by
many animals, including blue-ringed octopuses, pufferfish and certain South American
frogs.