4. a) You have the following information about the ratings of 10 firms over 3 years (Table 4.1). Estimate the one year transition probability matrix between ratings A, B, C and the default state D. Write the answer in a (10 marks) table with the structure as in Table 4.2 Firm ID 10 t-1 t#2 t-3 Table 4.1: Firms' ratings Rating the year end Rating at a year beginning Default Table 4.2: Transition Probabilities