23
(A) aroma
(B) animosity
(C) entity
(D) enigma

Animals possess sensory powers that humans lack. Homing pigeons fly with great speed and accuracy when   21   with messages to faraway places. How do pigeons   22   themselves in unfamiliar regions? This remains something of an   23   . The mystery, however, is partly explained by a pigeon’s ability to seeultraviolet light, which reveals the sun’s position even through clouds. In addition, pigeons can hear soundwaves that have traveled hundreds of miles. These waves   24   a pigeon’s sense of direction by indicatingdistant mountains and seas. Pigeons even appear to   25   changes in the earth’s magnetic field.