在测定种子发芽率时，选出了1000粒玉米种子，在适宜的条件下，有40粒种子未发芽，这一批种子的发芽率为（　　）

    Betty is an English girl. She studies in a middle school. On weekdays(非休息日) she gets up at 6:30. She has breakfast at seven. Then she goes to school on her bike(自行车). The school is near her home. She gets to school at around seven-thirty.

    Classes start at eight o'clock. She has four classes in the morning and two in the afternoon. Her favorite subject is history. She has lunch at 12:00 at school. Classes end(结束) at three-thirty. After school she usually goes to the history club and then goes home. She gets home at 5:00.

某NaNO₃溶液中混有少量NaC1、Na₂SO₄和Na₂CO₃杂质，为除去这些杂质，仅提供三种药品：AgNO₃溶液、BaC1₂溶液和稀HC1。请设计合理的顺序将杂质逐一除去，并填写下表：

关于美国的农业，正确的是（    ）