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Verbal·Standard English Conventions·Form, Structure, and Sense
easy
The formation of desert varnish—the dark coating found on exposed rocks in arid regions—is an exceptionally slow process, the coating typically        by only a few micrometers every thousand years as windblown clay particles and manganese oxides build up on rock surfaces.
Which choice completes the text so that it conforms to the conventions of Standard English?
A
thickens
B
will thicken
C
thickening
D
has thickened