Trilobites are index fossils because they a. changed slowly through time c. exhibit specific characteristics b. lived over large areas of the world d. all of the above select the best answer from the choices provided a b c d

Answer: Option D all of the above

Explanation:

Index fossils are the ones which are used to define as well as identify geological periods.Trilobites are one of the earliest known groups of arthropods.There are thousands of species of trilobites that are known and they are found in various parts of the world. The main reasons that trilobites are considered as index fossils because they were found to live in many regions of the world during specific period in the geologic time.They first appeared in the early Cambian period and then increased in number through the Palaeozoic era and finally disappeared at the end of the permian.Trilobites appeared quickly in the geologic time and then moulted just like the other arthropods and thus, are used as index fossils by scientists.

Thus, all the characteristics mentioned in the options are the reasons why trilobites are used as index fossils.