Input Output Table 3

Input Output
9 4.5
10 5
11 5.5
12 ?
13 6.5

Which rule describes the relationship between the input/output values shown on the table? Which number completes the table?

A) 2; subtract 3.5
B) 6; divide by 2
C) 6; subtract 0.5
D) 8; add 2.5