The correct answer is: E. was/appeared
The Cambrian explosion was a geologically rapid and relatively brief event in Earth’s history, about 541 million years ago, during which most major animal phyla appeared in the fossil record. The sudden appearance of complex life is often referred to as the Cambrian Explosion, although it is not known if this was an actual explosion of speciation or just an increase in the rate at which fossils were being deposited.
The Cambrian explosion is important because it marks the beginning of the Phanerozoic eon, which is the eon in which we currently live. The Phanerozoic eon is characterized by the presence of abundant fossils, which provide a wealth of information about the evolution of life on Earth.
The Cambrian explosion is also important because it marks the beginning of the diversification of animal life. Prior to the Cambrian explosion, there were only a few simple animal phyla. After the Cambrian explosion, there were many more animal phyla, including all of the major phyla that exist today.
The Cambrian explosion is a complex event that is still not fully understood. However, it is clear that it was a major turning point in the history of life on Earth.
The other options are incorrect because they do not accurately reflect the timing of the Cambrian explosion. Option A, “was/had appeared,” is incorrect because the Cambrian explosion was a geologically rapid event, not a gradual one. Option B, “has been/will appear,” is incorrect because the Cambrian explosion has already happened and will not happen again. Option C, “has been/appears,” is incorrect because the Cambrian explosion was a past event, not a present one. Option D, “is/appeared,” is incorrect because the Cambrian explosion was a past event, not a present one.