After which event could you say that evolution has occurred?
a. an animal gets fat
b. a white-eyed fly is born into a population of brown-eyed flies
c. an allele frequency changes in a population
d. a new predator comes into an environment willｇｉｖｅ ｍｅｄａｌ

C. An allele frequency changes in a population

Explanation:

Evolution refers to the accumulation of genetic variations in the gene pool of population leading to the origin of the better adapted population. Evolutionary agents such as mutations, natural selection, gene flow, etc. change the allele frequency in the gene pool of the population.  

The changed allele frequency of a population represents that fact that one or more of the above mentioned evolutionary agents are working on the gene pool and population has occurred.  

9.9 years

a = p(1 + rt) . . account balance after time t at rate r starting with principal p

3000 = 2500(1 + 0.0202t) . . filling in the given numbers

1.2 = 1 + 0.0202t . . divide by 2500

0.2 = 0.0202t . . . subtract 1

0.2/0.0202 = t ≈ 9.901

it will take about 9.9 years for the account balance to reach $3000.