There was a question about a portfolio having an average duration of 5.xx. Then there were 4 choices about what situation would make the estimated duration closest to the actual duration (or something like that).

a. Interest rates move 5 basis points for every bond in portfolio

b. Interest rates AVERAGE 5 basis points for all bonds in portfilo

c. Interest rates move 100 basis points for every bond in portfolio

d. Interest rates AVERAGE 100 basis points for all bonds in portfilo

选哪个？