A certain species of desert lizard digs tunnels in which to lay its eggs. The eggs must incubate inside the tunnel for several weeks before hatching, and they fail to hatch if they are disturbed at any time during this incubation period. Yet these lizards guard their tunnels for only a few days after laying their eggs. Which of the following, if true, most helps explain why there is no need for lizards to guard their tunnels for more than a few days? ['The eggs are at risk of being disturbed only during the brief egg-laying season when many lizards are digging in a relatively small area.', 'The temperature and humidity within the tunnels will not be suitable for the incubating eggs unless the tunnels are plugged with sand immediately after the eggs are laid.', 'The only way to disturb the eggs of this lizard species is by opening up one of the tunnels in which they are laid.', 'Each female lizard lays from 15 to 20 eggs, only about 10 of which hatch even if the eggs are not disturbed at any time during the incubation period.']