Reading 55, MBS, Q21 (Pg 421) - CFAI - I'm wondering security, average, manager, shorter, better

pacmandefense

Question: from the manager of saving&loan association portfolio, which MBS is better:
1. Passthrough security with a WAM of 310 months or
2. PAC tranche CMO with average life = 2years
Answer from book: choice 2 with shorter average life, due to less extension risk.
Why only extension risk?