Answer the following question given this paragraph: 

Exposed rocks in desert areas often develop a dark brown or black coating called desert varnish ( Figure below ). Wind transports clay-sized particles that chemically react with other substances at high temperatures. The coating is formed of iron and manganese oxides.


Q: What is the term for the dark brown or black coating found on exposed desert rocks, which is formed of iron and manganese oxides?

 Choices:

- desert patina

- desert shadow

- desert rust

- desert varnish

A:
desert varnish
