A 51\:cm rod is built from 5\:cm rods and 2\:cm rods. All of the 5\:cm rods must come first, and are followed by the 2\:cm rods. For example, the rod could be made from seven 5\:cm rods followed by eight 2\:cm rods. How many ways are there to build the 51\:cm rod?
Answer Choices
A. 5
B. 6
C. 7
D. 8
E. 9