[mikeash]

There's another option as well: have enough redundancy to have substantial forces survive even a massive first strike, absorb it, and then retaliate at leisure. This seems to be the US's approach:

https://www.armscontrol.org/act/1997_11-12/pdd

I imagine other countries with smaller arsenals may not have that option. Even your B may not necessarily be an option, if their command structure relies on centralized control and would be vulnerable to a decapitation strike.