In April, Grinnel Paving, Inc. acquired a large quantity of crushed stone on account with payment due in 90 days. The stone was used in May when Grinnel Paving, Inc. completed a large parking lot for a local shopping center. In early July, Grinnel Paving, Inc. paid the supplier from which the crushed stone had been purchased. In which month should Grinnel Paving, Inc. recognize the cost of the crushed stone as an expense? What accounting principle provides the justification for the answer?