The following passage is an excerpt from an academic text on astrobiology. The second sentence contains a nonessential appositive phrase. What punctuation mark should be inserted into the blank to correctly set off this phrase?
Answer:In the hyper-arid McMurdo Dry Valleys of Antarctica, astrobiologists have discovered endolithic communities, which are microbial ecosystems that survive inside the microscopic pores of translucent rocks. These resilient organisms—primarily lichens and cyanobacteria【—】 obtain their moisture from transient snowmelt and photosynthesize using light that filters through the mineral layers. This habitat demonstrates that life can persist under the most extreme terrestrial conditions, offering a potential model for past life on Mars.
Answer
An em dash (—)
The sentence includes the nonessential appositive phrase 'primarily lichens and cyanobacteria' to describe 'These resilient organisms.' In Standard English, nonessential elements must be set off from the rest of the sentence using a matching pair of punctuation marks: two commas, two em dashes, or two parentheses. Because the appositive phrase is introduced with an em dash (—), it must be closed with a matching em dash (—) before the main verb 'obtain' to maintain grammatical symmetry.
Step-by-Step Solution
Key Concept
Punctuation of nonessential elements requires matching pairs of commas, em dashes, or parentheses to set off the parenthetical information.