Select the most appropriate option to substitute the underlined segment in the given sentence. If there is no need to substitute it, select No substitution required.

Creatures began moving properly around 570 million years ago shortly before the Cambrian explosion sparked the first vertebrates.

A

the Cambrian explosion when sparked the first vertebrates.

B

No substitution required

C

the Cambrian explosion whom sparked the first vertebrates.

D

the Cambrian explosion who sparked the first vertebrates.