---Are you still working in Shanghai?

---No, I’m working in Hangzhou now. I ______ in Shanghai for three years.

A．worked

B．have worked

C．had worked

D．was working

“杂交水稻之父”袁隆平院士将野生水稻与普通栽培水稻多次杂交，培育出高产的杂交水稻新品种，利用的原理是

A．生态系统的多样性

B．染色体的多样性

C．基因的多样性

D．细胞的多样性