He works as an engineer now, but he _______ on a farm for three years.

A．has worked

B．worked

C．was working

D．had worked

某市政府机关工作人员张某，因涉嫌犯间谍罪被立案侦查，后被依法采取强制措施。下列说法正确的是（）。

A.本案应由公安机关立案侦查

B.本案应由人民检察院立案侦查

C.对张某的取保候审应由国家安全机关执行

D.对张某的取保候审应由公安机关执行