Identify whether each of the following items typically constitutes positive or negative evidence when a manufacturing entity assesses whether a valuation allowance is required or should be adjusted for its net operating losses. 

a. Product orders are increasing. 

b. Book income for the past three years totals to a negative amount. 

c. Investment assets held by the taxpayer show a realized gain. 

d. The industry in which the taxpayer operates is in a down cycle. 

e. The entity’s tax plan includes a switch from MACRS accelerated depreciation to straight-line.