The collective suicide of flying birds is a difficult mystery of nature. A popular saying today is: collective suicide of birds, mainly because the movement of the earth's magnetic poles makes them misaligned. Which of the following, if true, would most weaken the opinion of the above experts? ['A large number of poisonous gas clouds floating in the air are the culprits of bird death.', 'The collective death of the flying birds is usually an accident of nature.', 'Asuka kills shrimp and crabs that feed on toxic algae in the river and sea.', 'Volcanic eruption produces large quantities of sulfide and kills birds collectively.']