back - n. the part behind the front; ad. the other way from forward
    bad - ad. wrong; acting against the law; not good
    balance - v. to make two sides or forces equal
    ball - n. something round
    balloon - n. a device of strong， light material that rises when filled with gas lighter than air
    ballot - n. a piece of paper used for voting
    ban - v. to not permit; to stop; n. an official restriction
    bank - n. an organization that keeps and lends money