As British scientist Peter Whibberley has observed, “the Earth is not a very good timekeeper.” Earth’s slightly irregular rotation
rate means that measurements of time must be periodically adjusted. Speciﬁcally, an extra “leap second” (the 86,401st
second of the day) is ______ time based on the planet’s rotation lags a full nine-tenths of a second behind time kept by
precise atomic clocks.
added, whenever
added; whenever
added. Whenever
added whenever
Choice D is the best answer. The convention being tested is punctuation between a verb and a preposition. When, as in this
case, a verb (“is added”) is immediately followed by a preposition (“whenever”), no punctuation is needed.
Choice A is incorrect because no punctuation is needed between the verb and the preposition. Choice B is incorrect because
no punctuation is needed between the verb and the preposition. Choice C is incorrect because no punctuation is needed
between the verb and the preposition.