选项组是由一个组框及一组复选框、________按钮或______按钮组成。
参考答案：选项 切换
—It's 10 o'clock. I have to go now.
— But it's raining outside. Better not leave _____ it stops.[ ]
A. when
B. since
C. while
D. until
铵态氮肥中氮元素以（）的形态存在
A.硝酸根
B.铵离子
C.酰胺态
D.氨水
