从关系数据库中取出符合某些字符属性的操作是______。
A．选择
B．比较
C．连接
D．投影
参考答案：D
粉尘颗粒越小，越容易被水润湿.
The students of Senior 3 are now under great pressure for it will be just one year ______ they take the Entrance Examination.
A．since
B．after
C．before
D．when
