Each of 4 brothers (Yellow, Green, Brown, and Red) has 5 rectangular plots of land. Land plots are numbered 1 to 21 in the diagram below. One of the plots is empty (not owned by any of the brothers).
Each brother's land plots are orthogonally connected to his other land plots. To be "by" something means orthogonally OR diagonally connected. Ex.: 8 land plots are "by" the pond.
1. If the empty plot must be by the plots of a single (not more than one) brother, which land plot(s) can it be?
2. If the empty plot must be by the plots of multiple brothers' plots, which land plot(s) can it be?