9. Costa is building a new fence in his garden. He uses 25 planks of wood, each of which is $30 \mathrm{~cm}$ long. He arranges these planks so that there is the same slight overlap between any two adjacent planks, as shown in the diagram. The total length of Costa's new fence is 6.9 metres. What is the length in centimetres of the overlap between any pair of adjacent planks?
A 2.4
B 2.5
C 3
D 4.8
E 5