题目

    The boys were ready for an exciting week on the sea, but the trip did not start at all as they had ___21___.     It was the first day of their weeklong ___22___ journey on the Chesapeake Bay in Maryland, and they had already been ___23___ one storm. But that was ___24___ compared to the one they found themselves in ___25___ the same day.     Wave ___26___ wave was hitting their sailing vessel, and then, with a sudden surge, their ship—the Marguerite—was pushed onto its side.     A wall of wind and rain suddenly ___27___ us, Ethan Johnson says. The boat tipped ove17．下列有关生物变异的说法，正确的是 　　　 A．基因中一个碱基对发生改变，则性状一定会改变 　　　 B．环境因素可诱导基因朝某一方向突变 　　　 C．21三体综合征是由染色体结构变异引起的 D．自然条件下，基因重组可发生在有性生殖细胞形成过程中