21. (2004 National College Entrance Examination, Tianjin Paper) From 4 boys and 2 girls, 3 people are randomly selected to participate in a speech contest. Let the random variable $\xi$ represent the number of girls among the 3 selected people.
(I) Find the distribution of $\xi$;
(II) Find the mathematical expectation of $\xi$;
(III) Find the probability that "the number of girls $\xi \leqslant 1$ among the 3 selected people".