The word tenaciously in the passage is closest in meaning to:

For decades, materials scientists sought to replicate the remarkable ability of geckos to scurry vertically up smooth plate-glass surfaces and across ceilings without relying on chemical glues or suction cups. High-resolution electron microscopy revealed that gecko toe pads are carpeted with microscopic hair-like structures termed setae, which branch further into hundreds of nanoscale spatulae measuring mere fractions of a micron. When these millions of spatulae make intimate contact with a surface, weak van der Waals forces (electrostatic attractions between fluctuating electron clouds of adjacent molecules) multiply in aggregate into a formidable adhesive force. Crucially, gecko adhesion is direction-dependent: the bond remains tenaciously strong when sheared parallel to the surface, but detaches effortlessly with minimal peeling force when the toe angle tilts, allowing swift, energy-efficient locomotion.

Answer and explanation

Correct answer: B. firmly and persistently

  1. Step 1: Analyze the contextual meaning of "tenaciously" in the sentence: "...the bond remains tenaciously strong when sheared parallel to the surface..."
  2. Step 2: In this context, "tenaciously" describes a bond that holds tight with great firmness, resilience, and persistence against detachment force, making "firmly and persistently" the closest synonym.
Answer
firmly and persistently.

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