Which substantive audit procedure would be most likely to detect expired or worthless assets left on a company's books?

a$.$ Tracing acquisitions of long$-$lived assets to their original source documents.

b$.$ Noting the relative age of assets and idle equipment during a tour of the facilities and tracing the idle assets to the books.

c$.$ Comparing depreciation expense over time.

d$.$ Comparing current earnings with future expected earnings.