The weather is cold in early spring, and the ground is often covered with plastic film. The main purpose of this is (a) to promote the absorption of inorganic salts by seeds. B) ensure the seeds.

Seeds should not only have vitality, but also have suitable conditions to germinate under sufficient water, temperature and air. Some seeds also need light conditions. The temperature in early spring is low and unstable, and the germination of seeds needs suitable temperature. After sowing in early spring, plastic film is often used to cover, which can increase soil temperature, promote seed germination and promote early emergence.

Therefore, choose: B.