根任务组的种类属于 choice，这意味着用户能够选择任何子任务组并完成它。
The root task group is of the choice kind, which means the user can select any subtask group and complete it.
因为我们可以通过能给类发送的消息知道这个类，所以这意味着衍生类和基类属于同种类型。
Since we know the type of a class by the messages we can send to it, this means that the derived class is the same type as the base class.
跟其他生物一样，人类属于一类与其他动物有一些共同特征的种类。
Like all other living creatures, human beings belong to a group of other animals that share certain characteristics.
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