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) 1.75 2.25 What is the average age and the average depreciable life of the company's fixed assets at the end of 2018? a. Average age; Average depreciable life 1.6 years 7.2 years O b. Average age: Average depreciable life 1.6 years: 10.8 years Oc. Average age; Average depreciable life 4.5 years: 10.8 years O d. Average age: Average depreciable life 4.5 years: 7.2 years