7. The following annulus is cut into 14 regions. Each region is painted with one colour. What is the minimum number of colours needed to paint the annulus so that any no two adjacent regions share the same colours?
(A) 3 ;
(B) 4 ;
(C) 5 ;
(D) 6 ;
(E) 7 .
7. Answer: (B).
The left shows that 3 colours are not enough. The right is a painting using 4 colours.