Answer:
In the course of evolution mammals are the animals which are evolved recently as compared to other groups such as fish, amphibians, or reptiles.
In addition, fur or hairs on body, mammary glands, middle ear bone, warm-blood, et cetera are the characteristics of mammals.
These characteristics were evolved during the course of evolution; they were not present in ancestral organisms.
However, tail, gill pouches, et cetera are characteristics of our ancestral groups. Thus, characters can be sometimes observed in mammals.
But mammals characteristics can not be observed in organisms which were evolved before mammals such as fishes, amphibians, reptiles, et cetera.
The answer is that the criteria of classification change whith the improved understanding of organisms around us
I believe your answer is B, Because electrons are clinched to protons, most times, therefor there is not an electron microscope, but a ELECTRONIC Microscope will show the organism on a screen. hope that helped some<span />
