In polluted industrial English cities during the Industrial Revolution, two plant diseases -- black spot, which infects roses, and tar spot, which infects sycamore trees -- disappeared. It is likely that air pollution eradicated these diseases. Which one of the following, if true, most strengthens the reasoning above? ['Scientists theorize that some plants can develop a resistance to air pollution.', 'Black spot and tar spot returned when the air in the cities became less polluted.', 'For many plant species, scientists have not determined the effects of air pollution.', 'Black spot and tar spot were the only plant diseases that disappeared in any English cities during the Industrial Revolution.']