In 1972, French analysts at the Pierrelatte uranium enrichment plant discovered a bizarre anomaly in uranium samples from Gabon. The proportion of the fissile isotope uranium-235 was significantly lower than the standard value found everywhere else on Earth. Further investigation revealed that the Oklo deposits ______ had actually undergone self-sustaining nuclear fission reactions approximately two billion years ago.
Which choice completes the text so that it conforms to the conventions of Standard English?
- —which contained a unique combination of physical conditions—Answer
- B, which contained a unique combination of physical conditions—
- C—which contained a unique combination of physical conditions,
- D, which contained a unique combination of physical conditions
Answer
—which contained a unique combination of physical conditions—
The correct answer correctly uses a pair of em dashes to set off the nonessential relative clause modifying the subject. In English, nonessential elements must be bracketed by matching punctuation marks: either two commas or two em dashes. Since the clause is followed by the main verb of the sentence, wrapping the clause in em dashes successfully maintains grammatical symmetry.
Step-by-Step Solution
Key Concept
Punctuation of Nonessential Elements