When we analyze the advertising industry, we notice that a$.$ the industry is dominated by large advertising powerhouses in terms of total revenues b$.$approximately $70\%$ of all media dollars spent are handled by the top $500$ advertising agencies c$.$ approximately $30\%$ of all media spending is handled by the top $10$ advertising agencies d$.$Ogilvy and Mather is one of the largest $($top $5)$ advertising agencies in the world e$.$ all of the above