Over fifty percent of swallows living on cliffs in Nebraska were killed when a cold spell hit the area in 1996. Scientists collected nearly 2000 dead swallows from the base of the cliffs and captured about 1000 live ones. By measuring the body mass of these birds, they found that birds with larger than average body mass survived the cold spell better than the ones with smaller than average body mass. The data collected suggests that

• Correct answer
  A

  The population is undergoing natural selection.
• B

  The population is undergoing disruptive selection.
• Correct answer
  C

  There is directional selection acting on the population.
• D

  Stabilizing selection is acting on the population.

Hence, the correct answer is options: 'A', 'C'.