The photograph shows a sample of stained frog blood cells as viewed with the high-power objective of a compound light microscope, with a specific structure labeled "A". Which of the following is the BEST description for the structure labeled "A?"
A Chloroplast that carries out photosynthesis
B Lysosome that stores hydrolytic enzymes
DC Mitochondrion that produces ATP
OD Nucleus that regulates cell activity

Note: You will find the photograph in the attached files.

The Nucleus that regulates cell activity

Explanation:

The structures labeled with the "A" are the nucleus of erythrocytes.

Frog´s erythrocytes might be about 50μm, are ovoid and biconvex, and present a nucleus with condensed chromatin. They are stained red by eosin and the nucleus by hematoxylin. DNI is the stained material in the cell.    

affects the water bodies: excessive use of fertilizers causes eutrophication. the fertilizers which are used on plants affect the quality of land on which it is used, further causing erosion of soil. the fertilizers used on plants contain harmful chemical fumes which affect the environment.