Which is an example of how bacteria help keep the environment clean?
They can reduce oxygen in the air.
They can decompose waste.
They can help develop medicines.
They can slow digestion.

Which statements explains what the fossil records indicates? check all that apply. a) earth is 4.5 billion years old b) many organisms have become extinct c) mammals have always existed on earth d) mass extinctions have occurred several times on earth e) the amount of water on earth has decreased over time?