As we know, his father is a very famous scientist in physics. But what you don't know is that he _______ as a farmer for five years.
A：has worked
B：worked
C：had worked
D：works
项目
标准
感官指标
无异味、臭味等
化学指标
pH 6．5～8．5，铜<1．0 m9·L-1，铁<0．3 mg·L-1，
氟化物<1．0 mg·L-1，游离氯≥0．3 mg·L-1等
