'B', 'C' and 'D' have never seen 'A' writing, but they give opinion about 'A''s handwriting. Whose opinion of these is relevant?

• A. 'B' because 'B' is a businessman in Delhi. 'A' has written many letters addressing 'B'
• B. 'C' because 'S' is the clerk of 'A'. The duty of 'C' was to see and file the correspondence of 'A'
• C. 'D' because 'D' is the broker of 'C'. 'C' has put the letters written by 'A' to 'C' equal to 'D' and has taken the advice of 'D'
• D. all of the above