---The beef in this restaurant tastes nice.
--- You can't find better beef in any other restaurant.
一群氢原子处于同一较高的激发态,它们向较低激发态或基态跃迁的过程中( )
要使4y2+9是完全平方式,需添加一项,添加的项为____(写出一个答案即可).