扁形动物的主要特征是：身体呈____对称；背腹扁平；____．
The French book must belong to _______, because he studies French.
--- Oh, my god. I ______ 2 kg. this month.
--- Don't worry. It's normal for a growing teenage girl.
下列有关“低温诱导植物染色体数目变化”实验的叙述，正确的是：
