Index fossils are the remains of an organism that existed for only a short period of time and on a wide geographic range. This type of fossil is useful when it matches rock layers. This information is used by the paleontologists to identify the relative age of fossils and rock layers.

C. I and II only

The present time is under the Cenozoic Era. This era is known as the Age of Mammals.

A. Cenozoic

Endosymbiotic theory explains how eukaryotic cells are formed from prokaryotic organisms and their symbiotic relationship with each other. It tells how prokaryotic cells stayed and lived inside the eukaryotic cells to function as organelles. Therefore, D is the correct answer.

D. All organelles evolved from specialized enfoldings of the plasma membrane.

The photosynthetic bacteria that entered the eukaryotic cell became chloroplasts, whereas the aerobic bacteria which also entered the eukaryotic cells have evolved into mitochondria. This is why chloroplasts and mitochondria have similar structure of a bacterial cell and that includes having DNA and ribosomes similar to a bacteria. They are also using binary fission to reproduce during mitosis.

C. I and III only

Potassium-40 is useful for dating very old rocks because it has a long half-life of 1.26 billion years.

A. It has a very long half-life.

Invertebrates, such as brachiopods and trilobites, lived in the sea during the Cambrian period. Therefore, the Cambrian period is called as the Age of Invertebrates.

D. Age of Invertebrates

The radioactive isotope of Thorium-230 has a half-life of 75,000 years, which means that after 75,000 years, there will be a decay on the sample for about 50%. After that, if it reaches another 75,000 years, which is now 150,000 years in total, a quarter of the sample will undergo decay. Once it reaches 300,000 years, 100% of the sample will decay. Therefore, it would take 225,000 years to decay 7/8 of the sample.

B. 225,000 years

The radioactive isotope of potassium-40 has a half-life of 1,300 years, which means that after 1,300 years, there will be a decay on the sample for about 50% or 1/2. After 2,600 years, there will be a decay of about 25% or 1/4. After 3,900 years or three half-lives, there will be 12.5% decay or 1/8, and after 5,200 years or four half-lives, it will have 6.25% decay or 1/16.