The most likely explanation for the variation is the offspring were produced from different combination of genes. A single pair of gold fish mentioned in the question means a male and a female gold fish. The two of them mated and contributed different genes to the fertilized fish eggs, this results in production of various body shape and colouration which is known as variation.
The answer is C are limited to life in a single year
The infectious diseases are considered as emerging when their occurrences have increased in the past few years and could increase in the coming time like SARS, H1N1, or HIV/Aids.
On the other hand, the re-emerging infectious disorders are those, which were once considered as the major health issues globally or in a specific nation, and then got diminished drastically, however, are again turning into the health issues for a substantial ratio of the population like tuberculosis and malaria.
A is the answer
a replication fork
