SAT Question 56336696: Transitions answer and explanation

English - Transitions - Sequence Transition
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When using sumac for dyeing cloth, textile artists first soak it in water to release its color. Then, they add the cloth to the dyebath and simmer it for hours, perhaps even days. ______ they will remove the cloth, at which point it will have turned a vibrant red color.
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Difficulty:Easy
Which choice completes the text with the most logical transition?