题目

USA's participation in World War II brought major changes to the lives of American women. Before the war, there had been a huge pool of female labour, consisting largely of young, single women. Many jobs were closed to them and they were effectively restricted to working in domestic service and selling - goods business. Wartime production requirements, however, combined with the loss of the men who entered the military, provided women with the opportunity to move into a far wider range ofjobs. The response to the US government's request for more women workers was amazing and the changesm 诱变育种与杂交育种的不同之处是 ①能大幅度改良某些性状 ②能形成新基因型 ③能形成新基因 ④需要大量的选育工作 [　　] A．①② B．①③ C．②③ D．②④