Answers: C. expand and F. spread
Why: The sentence signals a reversal ("striking reversal... retreat") - so the blank must mean the opposite of retreating. The supporting clues confirm this: the ice is "adding measurable thickness" and moving "across formerly bare stretches of coastline," both pointing to growth.
- C. expand - directly means to grow or increase in extent, matching "adding measurable thickness."
- F. spread - means to extend over a larger area, matching "across formerly bare stretches of coastline."

These two are synonyms that both correctly complete the meaning, which is the key requirement for GRE Sentence Equivalence - the two correct answers must be interchangeable and produce logically equivalent sentences.

Why the others fail:
- A. contract - means to shrink; this is the opposite of what the sentence describes (and is essentially a synonym-trap paired with E , not the intended pair).
- B. sublimate - means to change directly from solid to gas; this would describe ice disappearing, contradicting "adding thickness."
- D. fracture - means to crack or break apart; unrelated to growth or retreat reversal.
- E. dwindle - means to gradually decrease; again the opposite of the reversal described, and it pairs logically with A (contract), which is a classic GRE trap - two words that seem to "go together" but both contradict the sentence.

Trap alert: Notice that A (contract) and E (dwindle) are synonyms of each other too - this is intentional. GRE Sentence Equivalence often includes a plausible-sounding synonym pair that fits the surface topic (ice, climate) but contradicts the actual logic of the sentence (a reversal of retreat means growth, not further shrinking). Always anchor to the logical signal word ("reversal") rather than just topical association.