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: highest concentration heavy metal in earth's crust
Useful Output:

Doc 1: Heavy metals such as Mg, Al, Si, K, Ca, Ti, Fe, V, Cr, Mn, Co Ni, Cu, Zn, As, Cd, Ba, La, and Pb were determined in twenty-six samples. The
Doc 2: Chromium, nickel, lead and zinc concentrations proved to exceed most likely compost, while arsenic and mercury concentrations remained usually
Doc 3: Non-biological essential heavy metals include lead (Pb), mercury (Hg), cadmium (Cd) and tin (Sn). They can be tolerated at low levels, but
Doc 4: Mafic igneous rocks generally contain higher concentrations of heavy metals, such as Cu, Zn, Cr, Co, Fe and Ni (He et al., 1998). In contrast,
Doc 5: 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.