Digital ACT Reading Practice Question #1746

As it is used in the passage, the word "elusive" most nearly means: Passage Excerpt (Natural Science): For over a century, the ability of migratory birds to navigate across continents with precision remained an enigma. While celestial cues and olfactory maps provide benchmarks, experiments under overcast skies demonstrated songbirds possess a compass sensitive to Earth's geomagnetic field. Classical biophysics hypothesized navigation was mediated by magnetite crystals in the beak. However, quantum biology has uncovered an alternative: radical-pair magnetoreception localized within cryptochrome proteins in the avian retina. When blue photons strike cryptochrome-4 (Cry4), an electron transfers along tryptophan residues, generating an entangled pair of radical electrons. The quantum spin state of this entangled pair is exquisitely sensitive to Earth's magnetic field inclination, overlaying visual navigation cues onto the bird's visual cortex.