A genetic algorithm is a variant of stochastic beam search, in which successor states are generated: 遗传算法是随机束搜索的一个变体，其中后继节点的生成是由：

A、by combining two parent states rather than by modifying a single state. 组合两个双亲状态而不是修改单一状态。

B、by modifying a single state rather than by combining two parent states. 组合两个双亲节点而不是修改单一节点。

C、by combining two parent nodes rather than by modifying a single node. 组合单一状态而不是组合两个双亲状态。

D、by modifying a single node rather than by combining two parent nodes. 组合单一节点而不是组合两个双亲节点。