求助Relational Databases

在coursera上学Introduction to databases，在Relational Databases这一讲遇到这样一道题（本讲的第一道题）：

Statement A: It can be useful to compose two selection operators.
   For example: σstate=`CA′(σenr>8000College)
Statement B: It can be useful to compose two projection operators.
   For example: πGPA(πsID,GPA,HSStudent)

答案是Both statements are false.
我觉得Statement A没有错，因为第一次σ选出的是College中enr>8000的表项，
而在此基础上加上一个state='CA'就是在第一次的选择结果中再选出state为CA的
表项，应该没有错误呀，为什么是false？求大神解答

坐等大牛解答  

求解答！！！

Coursera上就有Explanation了

Two selection operators in a row can always be replaced by a single selection operator whose condition is the "and" of the two selection conditions. If there are two projection operators in a row, the attribute list of the second (outer) projection must be a subset of the attribute list of the first (inner) projection. Thus, the first projection can be removed without changing the result of the expression.