$35 \cdot 20$ In a bag, there are 100 balls, among which there are 28 red balls, 20 green balls, 12 yellow balls, 20 blue balls, 10 white balls, and 10 black balls. If you randomly draw balls from the bag, to ensure that at least 15 balls of the same color are drawn at once, the minimum number of balls that need to be drawn from the bag is
(A) 100.
(B) 75.
(C) 68.
(D) 77.
(China Jiangsu Province Suzhou Junior High School Mathematics Competition, 1985)
［Solution］Consider the extreme case, where the balls drawn at once include: 
10 black balls, 10 white balls, 12 yellow balls, and 14 red, green, and blue balls each (at this point, all black, white, and yellow balls have been drawn), totaling 74 balls.
If one more ball is drawn at this point, then there will be at least 15 balls of the same color.
Therefore, the answer is $(B)$.