Doorknobs and other surfaces in clinical settings are often coated with or to prevent the transmission of microbes.

The correct answer would be copper, nickel, or zinc.

Explanation:

Heavy metals such as copper, zinc, nickel have antimicrobial properties due to which they are used in various places to prevent microbial growth. For example, copper sulfate is used in fish tanks and swimming pools to prevent algal growth. Metallic copper and/or copper linings in incubators also help in reducing microbial contamination.  

Coating of nickel or zinc is also used for the same purpose.

Thus, the correct answer would be copper, nickel, or zinc.

i think that d. wind is the answer.   i missed the question when i answered a. currents.

answer:

convection currents in the magma drive plate tectonics. heat generated from the radioactive decay of elements deep in the interior of the earth creates magma (molten rock) in the aesthenosphere. the aesthenosphere (70 ~ 250 km) is part of the mantle, the middle sphere of the earth that extends to 2900 km.