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The prebisch–singer hypothesis concludes that: a. technology lowers the cost of manufactured products, so developing countries should see an increase in their terms of trade. b. developing countries experience a long-run decline in their terms of trade, as the demand for primary products in higher-income countries declines relative to their demand for manufactured goods. c. because of unfair trading practices, labor in developing countries is exploited. d. opec has been responsible for a slowdown in the world's standard of living.