The question asks for the EXCEPT choice, meaning the one that does not explain sub-optimal prepayment behavior.

• Sluggish response to low interest rates can cause borrowers to miss the optimal time to refinance.
• Media effect can contribute to waves of refinancing when rate declines are widely publicized.
• Burnout effect means borrowers who have not refinanced after prior rate declines may become less likely to do so later.
• Sluggish response to high interest rates is not a problem from a valuation standpoint, because borrowers generally should not prepay when rates are high.

Therefore, the correct answer is B.