1. The division of Earth’s history into smaller units makes up the ____.

eras
geologic time scale***
periods
sequence of events

2. Today, some scientists think that ____.

birds evolved from dinosaurs
dinosaurs evolved from fish
birds evolved from amphibians
both a and b***

3. A life-form in the Precambrian time was ____.

cyanobacteria***
humans
reptiles
dinosaurs

4. Which trilobite characteristic did not change through time?

shape of eyes
three-lobed body***
number of body segments
number of spines

5. ____ existed during the Mesozoic era.

Angiosperms
Humans
Birds
both a and c***

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Eras

1. To answer the first question, "The division of Earth's history into smaller units makes up the ____.", we can utilize our knowledge of Earth's geologic time scale. The geologic time scale is a timeline that organizes Earth's history into different divisions based on major geological and biological events. These divisions include eons, eras, periods, and epochs. Therefore, the correct answer is "geologic time scale".

2. The second question states, "Today, some scientists think that ____." To determine the correct answer, we need to consider the proposed evolutionary relationships. Scientists currently believe that birds evolved from dinosaurs, as well as that dinosaurs evolved from fish. Therefore, the correct answer is "both a and b".

3. The third question asks about a life-form in the Precambrian time. To identify the correct answer, we must recall the types of life-forms that existed during that period. During the Precambrian time, cyanobacteria were present on Earth. Therefore, the correct answer is "cyanobacteria".

4. The fourth question inquires about a trilobite characteristic that did not change through time. Trilobites were a group of extinct marine arthropods. Among their characteristics, the shape of their eyes varied through time, the number of body segments changed, and the number of spines differed between species. However, the three-lobed body characteristic did not change. Therefore, the correct answer is "three-lobed body".

5. The fifth question asks about what existed during the Mesozoic era. The Mesozoic era is known as the "Age of Reptiles" and is characterized by the dominance of dinosaurs. However, it was also during this era that angiosperms (flowering plants) and birds appeared. Humans, on the other hand, did not exist during the Mesozoic era. Therefore, the correct answer is "both a and c", referring to angiosperms and birds.