In his victory speech，______ before a huge crowd of his supporters，Obama declared that “change has come to America”．
已知x，y，z均为正数，且x+y+z=2，则x+2y+3z的最大值是（ ）
观察和实验是生物学家科学探究的基本方法，下列有关叙述正确的是（ ）
与酿制酸奶有关的微生物是（ ）
