The fossil record shows that two groups of organisms are related, which means they share

a common ancestor

Explanation:

When we have fossils that are very similar, exhibit the same or very similar traits, and the similarities between the fossils become greater as we go back in time, that means that the organisms are related, or rather that they share a common ancestor. The evolution works in a manner where we have one species, but because of changes in the environment, some members of the species will develop new characteristics in order to survive, thus they will become new species. As the time goes on, the species that came to be from one single species will start to look less and less alike because of the environment and what is advantageous for them to be more competitive. We can take the lynx and the smilodon for example. Both of them are part of the cat family (felidae), and they share a common ancestor around 20 million years ago, but gradually their ancestors have developed different traits, through the basic ones remained the same, with the end product being two animals from the same family that look very different.

google my dude. if it doesn't show up ask me.

b. the carbon cycle ensures living things have a source of hydrogen