A plausible explanation of the disappearance of the dinosaurs is what is known as the comet theory. A large enough comet colliding with Earth could have caused a cloud of dust that enshrouded the planet and cooled the climate long enough to result in the dinosaurs' demise. Which one of the following statements, if true, most seriously weakens the argument? ['Many other animal species from the era of the dinosaurs did not become extinct at the same time the dinosaurs did.', 'It cannot be determined from a study of dinosaur skeletons whether the animals died from the effects of a dust cloud.', 'The consequences for vegetation and animals of a comet colliding with Earth are not fully understood.', 'Various species of animals from the same era as the dinosaurs and similar to them in physiology and habitat did not become extinct when the dinosaurs did.']