2. Divide each face (a total of six faces) of a $3 \times 3 \times 3$ cube into 9 equal smaller squares (a total of 54 smaller squares). Now, color these smaller squares using red, yellow, and blue such that the maximum number of smaller squares sharing a common edge is ( ).
(A) 28
(B) 27
(C) 22
(D) 20
(1989, "Zu Chongzhi Cup" Junior High School Mathematics Invitational Competition)
To consider that small squares on the same face with a common edge cannot be the same color, and also to consider that small squares on adjacent faces with a common edge cannot be the same color. Since each small cube can only have one face painted red, the maximum number of small squares that can be painted red is 26, thus eliminating options (A) and (B). Also, 22 is achievable, therefore, the answer is (C).