Every year, Hercules Inc. has invested an increasing amount in manufacturing equipment. Given that there are a large number of relatively inexpensive machines and all the machines have relatively similar useful lives, Hercules has depreciated the machinery as a group at a uniform rate using the straight-line method. Over time, the ratio of the group's total accumulated depreciation to the total cost of the machinery has increased steadily. Currently, the ratio stands at 0.75 to 1. What is the most likely reason for this increasing ratio? O The estimated average useful life of Hercules' machinery is greater than its actual average useful life. O The estimated average useful life of Hercules' machinery is equal to its actual average useful life. O Hercules has been retiring fully depreciated machinery that should have remained in service. O The estimated average useful life of Hercules' machinery is less than its actual average useful life