Which of the following organisms have some strains that naturally carry a circular, 6.3-kb, 2-micron plasmid? A. Escherichia coli
B. Saccharomyces cerevisiae
C. Agrobacterium tumefaciens
D. Drosophila melanogaster

B

Explanation:

The correct answer for the organism with strains that naturally carry a circular, 6.3-kb, 2-micron plasmid would be Saccharomyces cerevisiae.

Most strains of Saccharomyces cerevisiae have been found to possess extrachromosomal elements in their nucleus. This extrachromosomal element is circular in nature and of about  2 microns and 6.3 kb without any known specific advantage or function.

Due to the simple nature of the yeast's plasmid, it has been a major source of study for the understanding and control of how replication works in eukaryotic organisms.

The correct option is B.

estuaries are the partially enclosed coastal bodies of brackish water formed at the mouth of sea where river meets an ocean. it is a transition zone between river and sea water so estuaries are influenced to both marine environment such as tides, waves and to riverine such as flow of fresh water and sediments. the mixing of sea and fresh water provide high level of nutrients, making estuaries a high product habitats.

many estuaries suffers degradation from a variety of human activities like erosion from deforestation, overgrazing, overfishing, drainage and eutrophication.

so from the above statement answers will be

1. false as estuaries are formed due to both brackish and freshwater.

2.true, due to high productivity estuaries are breeding ground for many marine species.

3. direct and indirect activities of man that contribute to pollution and deposition of sediments.