光斜射到平面镜上，入射光线与镜面的夹角为30°，反射角为（　　）

质量相同的甲、乙、丙、丁4个小球，分别静止在水中的不同深度处，如下图所示，则这4个小球在水中所受浮力最小的是（  ）

阅读下列短文，从每小题的三个选项中选出最佳答案

      It is morning now. Look at the sun, it rises in the east(东方). Look at the tree, the shadow is long. It is noon now. The sun is high in the sky. It is very hot. Look at the tree. The shadow is short. It is evening now. The sun goes down in the west(西方). Look at the tree, the shadow is long again. It is night now. The moon is high in the sky. There are some stars in the sky too.

1. 若把麻雀、带鱼、蝴蝶、蚯蚓、蝙蝠、青蛙、猪、鲸划分为有翅动物和无翅动物，其分类的标准是____ 。若把这些动物划分为有羽毛动物和无羽毛动物，其分类的标准是____ 。若把这些动物划分为胎生动物和卵生动物，则其分类的标准是____ 。