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Explanation: Index fossils are useful because they tell the relative ages of the rock layers in which they occur. Geologists use particular types of organisms, such as trilobites, as index fossils.
A physical change<span> in a substance doesn't </span>change<span> what the substance is. In a </span>chemical change<span> where there is a </span>chemical reaction<span>, a new substance is formed and energy is either given off or absorbed.</span>
Base pairs are more biology but they are what make up DNA there are 4 (adenine, thymine, guanine, and cytosine) they each pair with each other (A to T and G to C).
