Select the correct statement about current hypotheses for the cause of the Cambrian explosion. A. Oxygen levels reached a critical threshold at the start of the Cambrian explosion, making the evolution of big, mobile animals possible.
B. With the evolution of algae, oxygen was produced by oxygenic photosynthesis for the first time in the history of life on Earth.
C. Key developmental toolkit genes arose early in the Cambrian, leading to the Cambrian explosion of animal phyla.
D. There are no plausible hypotheses accounting for the rapid adaptive radiation known as the Cambrian explosion.

A. Oxygen levels reached a critical threshold at the start of the Cambrian explosion, making the evolution of big, mobile animals possible.

Explanation:

The Cambrian explosion was an event that happened approximately 541 million years ago and it lasted for about 13 – 25 million years.

The current hypothesis for the cause of the Cambrian explosion is that oxygen levels reached a critical threshold at the start of the Cambrian explosion, making the evolution of big, mobile animals possible.

Option A. is correct.

the statements that apply here about binary star system are

"have stars that might appear to wobble"

"often have one star that is brighter than other"

a binary star is a star system consisting of two stars revolving around their common barycenter. systems of two or more stars are called multiple star systems.