You want to examine the effects of advertisement on firms' profitability using data of about 6,000 firms in 50 industries in 2018 and 2019. You have the following variables in the data: return on assets (roa), a dummy variable for whether firms advertise (advert), total assets (assets), number of workers (workers), the year, and an identifier of the firms. (a) Present and describe a fixed-effect model to examine the relationship. (b) Highlight the key identifying assumption of the model in Part (a) and discuss whether you think that assumption is likely to hold.