According to the situation described in the poem, it is analyzed that "Si Er is long day and night, and the voice of begging is diligent. The caterpillar is not easy to catch, and the yellow mouth is not full. " The swallow's chick should be () A.
During the breeding season, young birds eat a lot, and parents often need to feed them dozens or even hundreds of times a day to ensure their growth and development. The yellow mouth refers to the young birds to be fed. According to the analysis of the situation described in the poem "The Yellow Mouth is Not Full", the young bird of the swallow should be a late bird.
So choose: B.