如图是生长素作用与其浓度关系图.课本上说:“一般地说,低浓度促进植物生长,高浓度抑制植物生长.”看图理解这句话,回答下列问题:
如图,正六边形ABCDEF的边长为2,则对角线AC= ____
By mentioning Muhammad Ali's words, the author intends to tell us that.