nikitathegreat
GMATNinja - Can you please explain the meaning of the sentence and also how ans choice B is ok without having ,+ and between two IC.
Choice (B) actually only has one independent clause: "[Savings banks] sometimes lose money."
After the "when", we have two parallel subject+verb pairs: 1) "short-term rates rise" and 2) "they are unable to raise the rates...". But these aren't independent clauses, because they include "when." The phrases "when short-term rates rise" and "when they are unable to raise the rates on their mortgages" can't stand alone as sentences, and they're not independent clauses.
So, the "and" in choice (B) is used to connect two parallel subject+verb pairs, but since the pairs aren't actually independent clauses, there's no need for a comma.
I hope that helps!