$35 \cdot 17$ In a wooden box, there are 3 red balls, 5 yellow balls, and 7 blue balls. If you take balls with your eyes closed, to ensure that the balls taken out have two of the same color, the minimum number of balls to be taken out is 
(A) 2.
(B) 4.
(C) 6.
(D) 8.
(China Guangzhou, Wuhan, Fuzhou, and other five cities junior high school mathematics league, 1991)
[Solution] Consider the extreme case: taking out 3 balls does not necessarily ensure that there are two balls of the same color, because the 3 balls can be of different colors.
At this point, if another ball is taken, then it can ensure that two balls of the same color are obtained.
Therefore, the answer is $(B)$.