They are called microscopic anatomy as the cell, tissues and organs are internal and needs a microscope for it to be seen inside the body.
They are considered so because are the indispensable parts of the nature cycle.Without formation,decay cannot be possible and similarly,formation cannot be possible without decay.In addition to this,THEY ARE IN A CHRONOLOGICAL MANNER SET BY NATURE and that's why cannot be seperated and so together constitute the cycle of nature.
The answer is A I believe
