Assisting

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Fossils are one of the most important sources of information about the Earth's past. They can tell us the age of the rocks in which they are found, what the environment was like when the fossilised organisms were alive, and even how the organisms functioned. They can also tell us about Earth movements, such as mountain building, about the former positions of continents (ancient geography), and about the evolution of life on Earth. Some of these uses for fossils are of economic importance, assisting in the search for oil and minerals.

I would like to know the meaning of "a". (a: assisting in the search for oil and minerals.)

1. and they are assisting in the search for oil and minerals.

2. and they assist in the search for oil and minerals.

3. as they assist in the search for oil and minerals.