. Although the population of a certain salmon species in the Pleasant River

has not decreased in the last five years, the average size of adult female

salmon of that species in the river has decreased significantly in this time.

The latter fact indicates that the population of this species is ultimately

endangered, since small fish of a given species lay far fewer eggs than

do large fish of the same species.

Which of the following is an assumption on which the argument depends?

     A  Excluding fish of the salmon species, the average size of fish in the

       Pleasant River has increased over the past five years. 

     B The largest adult female salmon living in the Pleasant River five

       years ago were significantly larger than the largest adult female

       salmon living there now. 

     C Smaller salmon are not an easier prey for salmon predators than are

       larger salmon. 

    D  The total number of eggs laid each year by salmon of the species

       in the Pleasant River has not varied significantly over the past five

       years. 

    E  The proportion of the eggs laid by small salmon of the species that

       hatch into fish that reach reproductive maturity is not significantly

       greater than that of the eggs laid by large salmon of the species. 

答案E