Why do immunized bond portfolio still have to be rebalanced periodically?

a.

To ensure the duration of the assets is greater than the duration of the liabilities.

b.

To ensure dedication of the portfolio when there are different liability end dates.

c.

To ensure that the fund only purchases investment grade bonds.

d.

Portfolio duration does not change linearly with time.

e.

Portfolio managers often do not have the required math skills.