Intervals of geologic time are bound by important events, like mass extinctions or the appearance of animals with hard parts. The first single-celled organisms appeared abo billion years ago. Many sea animals first show up in the Cambrian period of the Paleoz era. That was about 545 million years. It is sometimes referred to as the Cambrian exple because it happened relatively quickly. It lasted just over 50 million years. The next per the Ordovician, ended with mass extinctions of \( 60 \% \) of all marine invertebrates. Dinosa wouldn't appear for over 200 million years, during the Jurassic period in the Mesozoic e Based on the passage, in geology, 50 million years is A an era. B not considered a very long time. C too short for any significant events to occur. D about how long dinosaurs were on the planet.

Based on the passage, **50 million years** is described in the context of the **Cambrian period**, which is a subdivision of an era in geological time. The Cambrian period lasted just over 50 million years and is noted for the **Cambrian Explosion**, a relatively rapid event in geological terms where many sea animals first appeared. Let's evaluate the options: - **A. an era.** - Incorrect. An era is a larger division of geological time than a period. For example, the Cambrian is a period within the Paleozoic era. - **B. not considered a very long time.** - Correct. The passage describes the Cambrian Explosion as happening "relatively quickly," implying that 50 million years is not considered a very long time in the context of geological time scales. - **C. too short for any significant events to occur.** - Incorrect. The Cambrian Explosion was a significant event that occurred within this 50-million-year period. - **D. about how long dinosaurs were on the planet.** - Incorrect. The passage mentions that dinosaurs appeared during the Jurassic period and were present for over 200 million years, which is significantly longer than 50 million years. **Answer:** **B not considered a very long time.**