In the United States, less than half as many multifamily housing units were produced in the 1990s than in each of the previous two decades.
                    
                
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                Aless than half as many multifamily housing units were produced in the 1990s than
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                Bless than half as many multifamily housing units had been produced in the 1990s as
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                Cthere were less than half as many multifamily housing units produced in the 1990s than
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                Dfewer than half as many multifamily housing units were produced in the 1990s as
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                Efewer than half as many multifamily housing units had been produced in the 1990s than
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        正确答案: D