Every year people celebrate the Spring Festival in China．Usually it is in January or February．It's the most important(重要的)festival in China．So before it comes，everyone buys many things，and they often make a special kind of food called dumplings-It means"come together"．Parents always buy new clothes for their children and children also buy presents for their parents．On the Spring Festival eve(除夕)，all the family members come back home. They sing, dance and play cards．When they enjoy the meal，they give each other the best wishes for the coming year．They all have a good time。

牵牛花中，叶子有普通叶和枫形叶两种，种子有黑色和白色两种。现用普通叶白色种子纯种和枫形叶黑色种子纯种作为亲本进行杂交，得到的F₁为普通叶黑色种子，F₁自交得F₂，结果符合基因的自由组合定律。下列对F₂的描述中错误的是（    ）

淮安市汇通批发市场曾发生一场特大火灾。据查，火灾是由一个未熄灭的烟头引发的。那么未熄灭的烟头对发生这次特大火灾所起的作用是（）