A company using straight-line depreciation has an ending gross investment in fixed assets of $ 750, an accumulated depreciation of $ 300, and an annual depreciation expense of $150. The average depreciable life of the fixed assets and the average age of the plant and equipment are :Depreciable life Avg age of P&E（）

①A. 2 years         5 years  

②B. 5 years         2 years  

③C. 7 years         5 years