The mid-quarter convention must be used when the aggregate basis of assets (excluding buildings) acquired during the last 3 months of the year ... a. exceeds 40% of the aggregate basis of assets (excluding buildings) purchased during the year. b. is no more than 40% of the aggregate basis of assets (excluding buildings) purchased during the year. c. exceeds 40% of the aggregate basis of assets (excluding buildings) purchased during the first three quarters of the year. d. is no more than 40% of the aggregate basis of assets (excluding buildings) purchased during the first three quarters of the year