She doesn't know how to spend her summer holidays. Let's help her some good ideas.
近来市场上盛行一种绿色食品——桑椹,它是桑树的果实;在生物的结构层次上它属于
蝙蝠能够飞行,但不属于鸟类,而是属于哺乳类.主要由于是( )