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, ______
when their COF drops below 0.01, two surfaces reach superlubricity.
two surfaces, when their COF drops below 0.01, reach superlubricity.
reaching superlubricity occurs when two surfaces’ COF drops below 0.01.
superlubricity is reached when two surfaces’ COF drops below 0.01.
Choice D is the best answer. The subject of the modiﬁer “a nearly frictionless state” is “superlubricity.” Subject-modiﬁer
placement requires a modiﬁer and its subject to be next to one another, so “superlubricity” must be the ﬁrst word in the
missing clause.
Choice A is incorrect. This choice creates a subject-modiﬁer placement error. The subject of the modiﬁer “a nearly
frictionless state” is “superlubricity.” Subject-modiﬁer placement requires a modiﬁer and its subject to be next to one another,
so “superlubricity” must be the ﬁrst word in the missing clause. Choice B is incorrect. This choice creates a subject-modiﬁer
placement error. The subject of the modiﬁer “a nearly frictionless state” is “superlubricity.” Subject-modiﬁer placement
requires a modiﬁer and its subject to be next to one another, so “superlubricity” must be the ﬁrst word in the missing clause.
Choice C is incorrect. This choice creates a subject-modiﬁer placement error. The subject of the modiﬁer “a nearly
frictionless state” is “superlubricity.” Subject-modiﬁer placement requires a modiﬁer and its subject to be next to one another,
so “superlubricity” must be the ﬁrst word in the missing clause.