The mid$-$month convention applies to $..\mathrm{.}14.$a$.$ assets purchased during the last $3$ months whose aggregate basis exceeds $40\%$ of the aggregate basis of assets purchased during the year.b$.$ assets other than buildings purchased during the last $3$ months whose aggregate basis exceeds $40\%$ of the aggregate basis of assets other than buildings purchased during the year.c$.$ buildings.d$.$ buildings and land.