查询所有年龄在 20 岁以下的学生姓名及其年龄 以下正确的是()

A．SELECT Sname,Sage FROM Student WHERE Sage<=20;

B．SELECT Sname,Sage FROM Student WHERE NOT Sage<20;

C．SELECT Sname,Sage FROM Student WHERE NOT Sage>20;

D．SELECT Sname,Sage FROM Student WHERE Sage<20;

请帮忙给出正确答案和分析，谢谢！