For some of the fixed assets of a business, the balance in Accumulated Depreciation is exactly equal

Question:

For some of the fixed assets of a business, the balance in Accumulated Depreciation is exactly equal to the cost of the asset.
(a) Is it permissible to record additional depreciation on the assets if they are still useful to the business? Explain.
(b) When should an entry be made to remove the cost and the accumulated depreciation from the accounts?