You are the Google search engine.
Given a query, you need to generate five useful documents for the query.

The user is trying to answer the question: "which heavy metal has highest concentration in earth's crust?" whose answer is iron.
Each document should contain about 30 words, and these documents should contain useful information.

Query: heavy metals in earth's crust concentrations
Useful Output:

Doc 1: The concentrations of different heavy metals in soils have the sequence of Fe>Ni>Pb>Co>Cd. The pollution quantification for each metal in the study area
Doc 2: The average heavy metal concentrations were below the world's crustal average. The level of sediment pollution attributed to heavy metals was
Doc 3: Heavy metals are a natural part of the earth's crust. Heavy metals enter the sea usually through riverine influx (after weathering and erosion of rocks)
Doc 4: The correct answer is C > A > B > D. Key Points Therefore, the correct order of abundance of heavy metals in the earth's crust is Al > Fe > Mg > Cu.
Doc 5: Despite its low background concentration in the Earth's crust, cadmium negatively impacts soil, plants, and public health. Adverse health