SAT Question 359902ae: Words In Context answer and explanation

English - Words In Context - Meaning Of Phrase In Literary Prose
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The following text is adapted from Nathaniel Hawthorne’s 1837 story “Dr. Heidegger’s Experiment.” The main character, a physician, is experimenting with rehydrating a dried flower. At first [the rose] lay lightly on the surface of the fluid, appearing to imbibe none of its moisture. Soon, however, a singular change began to be visible. The crushed and dried petals stirred and assumed a deepening tinge of crimson, as if the flower were reviving from a deathlike slumber.
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Difficulty:Medium
As used in the text, what does the phrase “a singular” most nearly mean?