Bird lovers know that a good place to go bird-watching is Guandu Nature Park. As many as 220 kinds of birds come to this park every year. Since the park offers birds lots of food to eat and places to build their nests, it has become a paradise（天堂）for birds.

    Guandu Nature Park is also a great outdoor classroom. The park's Nature Center provides guided tours to the public. During the tour, visitors can learn about the environment. So, if you want to have a fun and educational weekend, come to Guandu Nature Park. Get away from city life and head into nature.

Which picture means "it's safe to land"?

在一对相对性状的杂交实验中，子一代（F₁）未表现出来的亲本性状称为（    ）