单项选择题

When an art museum wants a new exhibit, it buys things in finished form and hangs them on its walls. When a natural history museum wants an exhibit, it often must build it realistically— from a mass of material and evidence brought together by careful research.
An animal, for example, must first be skinned. Photographs and measurements are used to determine the animal"s structure in a natural position fighting, resting, or feeding. Then muscle forms are built and a plaster shell is made. Finally the skin is pulled over the shell like a wet glove. This completes the animal subject.
Displaying such things as stone heads, giant trees, and meteorites (陨石，陨星) is basically mechanical. Most other natural history exhibits present more difficult problems. For instance how can a creature be exhibited when it is too small to be seen clearly In these cases larger-than-life models are built. The American Museum of Natural History has models of fleas (跳蚤), houseflies and a myriad (无数的) of other insects enlarged up to seventy-four times. The models show the stages of the insects" development and the workings of their bodies. It is implied but not clearly mentioned in the passage that ______.