Research Has Demonstrated All but Which of the Following with Regard

Research has demonstrated all but which of the following with regard to people who speak more than one language?

A) People who are bilingual from an early age show an increase in the density of the gray matter in the lower left parietal area of the brain, suggesting that learning multiple languages actually promotes cognitive growth.
B) Adults who learn a second language later in life actually use different cognitive strategies than children who learn the second language, and thus their brains process the second language differently than people who learned a second language early in life.
C) Animals who are able to learn to use sign language are, in fact, able to learn multiple forms of sign language, which suggests that bilingualism is not a skill that is unique to human beings.
D) Asian immigrants who came to the United States of America as young children understood American English as proficiently as native speakers.