4. Since we know that price falls but quantity remains relatively unchanged, supply had to increase and demand had to decrease. Choice (b) states that the manufacturing costs of cassette tapes have fallen which would lead to an increase in the supply; and the price of CDs (a substitute for prerecorded cassettes) has fallen, leading to a decrease in demand.

   Let's consider the other choices:

  1. Manufacturing costs of cassette tapes have fallen and the price of compact disks (CDs) has risen. This would lead to an increase in supply (which is correct) but an increase in demand as well. These two together would cause quantity to rise unambiguously.
  2. Manufacturing costs of cassette tapes have fallen and the price of CDs has fallen.
  3. Manufacturing costs of cassette tapes have risen and the price of CDs has fallen. This would reduce supply and reduce demand, causing an unambiguous reduction in quantity.
  4. Manufacturing costs of cassette tapes have risen and the price of CDs has risen. This would reduce supply and increase demand, causing an unambiguous increase in price.
  5. Manufacturing costs of cassette tapes have fallen and the price of tape decks has fallen. This would lead to an increase in supply (which is correct), but an increase in demand (since tape decks are complementary goods for prerecorded cassettes) as well. These two together would cause the quantity to rise unambiguously.

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