A mass of "black water" containing noxious organic material swept through Laurel Bay last year. Some scientists believe that this event was a naturally occurring but infrequent phenomenon. The black water completely wiped out five species of coral in the bay, including mounds of coral that were more than two centuries old. Ttherefore, even if this black water phenomenon has struck the bay before, it did not reach last year' s intensity at any time in the past two centuries. Which one of the following is an assumption required by the argument? ['Older specimens of coral in the bay were more vulnerable to damage from the influx of black water than were young specimens.', 'Every species of coral in the bay was seriously harmed by the mass of black water that swept in last year.', 'The mounds of centuries-old coral that were destroyed were not in especially fragile condition just before the black water swept in last year.', 'Masses of black water such as that observed last summer come into the bay more frequently than just once every two centuries.']