Mashaal Masha
In 2015, a team led by materials scientists Anirudha Sumant and Diana Berman succeeded in reducing the coefficient of friction (COF) between two surfaces to the lowest possible level--superlubricity. A nearly frictionless (and, as its name suggests, extremely slippery) state, ________

Which choice completes the text so that it conforms to the conventions of Standard English?
Difficulty: Hard
A: 

when their COF drops below 0.01, two surfaces reach superlubricity.

B: 

two surfaces, when their COF drops below 0.01, reach superlubricity.

C: 

reaching superlubricity occurs when two surfaces' COF drops below 0.01.

D: 

superlubricity is reached when two surfaces' COF drops below 0.01.

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