Answer: Left Ventricle!
Explanation: The left ventricle receives oxygenated blood from the left atria and pumps it through the aorta into systemic circulation to supply the tissues of the body with oxygen.
Oak Bark was the first cell viewed by the light microscope
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.