Cannot determine why breaker is tripping
Just to close this question out, I followed the advice in the comments and did a stepwise disconnection of the various outlets and whatnot.  This led me to suspect the ceiling fan in bedroom B.  When I inspected its wiring, I noticed one of the ground wires was disconnected from the bundle it was tied-to (using one of hose colored plastic twisty things that look like a thimble).  I fixed that and voila, problem solved!  Thanks for all the comments and tips.