Complete the passage by entering the punctuation mark that correctly sets off the nonessential element in the sentence.
Answer:In 1928, Scottish biologist Alexander Fleming discovered penicillin, the world's first effective antibiotic, quite by accident. While studying *Staphylococcus* bacteria in his London laboratory, Fleming noticed that a contaminated Petri dish—which had been left unattended near an open window 【—】 had grown a mold that successfully inhibited the growth of the surrounding bacteria.
Answer
An em dash (—) is required to close the nonessential relative clause and match the opening em dash.
The sentence features a nonessential relative clause ('which had been left unattended near an open window') that interrupts the flow between the subject ('Petri dish') and its verb phrase ('had grown'). Standard English requires nonessential elements to be set off with matching punctuation. Because the clause is introduced with an em dash, a matching em dash must be used to close it.
Step-by-Step Solution
Key Concept
Punctuation of Nonessential Elements
Estimated Time:1m 0s