A company has the following information for the years ending December 31, 2017 and 2018. 2017 2018 Gross investment in fixed assets ($ mil) $3.6 $3.6 Accumulated depreciation ($ mil) 2.25 What is the average age and the average depreciable life of the company's fixed assets at the end of 2018? 1.75 a. Average age; Average depreciable life 1.6 years, 7.2 years b. Average age; Average depreciable life 1.6 years, 10.8 years O c. Average age: Average depreciable life 4.5 years; 10.8 years O d. Average age: Average depreciable life 4.5 years, 7.2 years