Why is the horseshoe crab an example of a living fossil?

a. there are fossils of extinct ancestors of horseshoe crabs, and the species is still alive today.
b. it hasn't changed since it first appeared on earth.
c. the structures in the horseshoe crab don't decompose, so complete fossils are discovered.
d. it's often found frozen to death and free of decomposition.

Horseshoe crab is an example of living fossil because there are fossil extinct of ancestors of horseshoe crabs, and the species still alive today. Horseshoe crab has been around for so long; it has survived mass extinction and it is still around.  

Horseshoe crab is considered living fossils because they have existed unchanged for about 445 million years. They are usually not crab; they are not closely related to lobsters or crab. Horseshoe crab is very closely related to spiders and other arachnids.

Generally, there are four species of horseshoe crab that is living today but one out of the species, called Limulus polyphemus can be found in North America. It is found near the Atlantic and gulf coast: from Maine to Mexico.

Horseshoe crab looks is almost the same with their ancient relatives despite the fact that they have been around for so long.  Their body structure is tank-like, which is made of:

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the element that provides strength to the exoskeletons of clams and oysters is phosphorus. this is so because calcium carbonate is the main component in the shells.

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