Article: finding sacagawea

which sentence expresses a central idea of the article?

a. "our family owned only an encyclopedia and a well-used dictionary." (paragraph 2)

b. "the presence of a mother and child was a sign that the expedition was a peaceful one an keep it safe from indian attack." (paragraph 5)

c. "my father said he didn't know, but he'd wondered himself if we had a common ancestor." (paragraph 9)

d. "we explored tiny libraries on american indian reservations." (paragraph 10)