The major companies upon which the Hollywood studio system was built had been resistant to the B-movie trend, but they soon adapted. All ultimately established "B units" to provide films for the expanding second-feature market. Block booking increasingly became standard practice: in order to get access to a studio's attractive A pictures, many theaters were obliged to rent the company's entire output for a season. With the B films rented at a flat fee (rather than the box office percentage basis of A films), rates could be set that essentially guaranteed the profitability of every B movie. Blind bidding, which grew in parallel with block booking, meant that the majors didn't have to worry much about the quality of their B's—even when booking in less than seasonal blocks, exhibitors had to buy most pictures sight unseen. The five largest studios—MGM, Paramount, Fox Film Corporation (Twentieth Century Fox as of 1935), Warner Bros., and RKO Radio Pictures (descendant of FBO)—had the additional advantage of being part of companies that also owned sizable theater chains, further securing the bottom line.