An artery a. brings blood back toward the heart b. has valves that prevent blood from flowing backward c. usually (but not always) carries oxygen-rich blood d. has thin, slightly elastic walls

An artery .

c. usually (but not always)  carries oxygen-rich blood

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".