用下面的语句创建了Student表: CREATE TABLE Student(Sno CHAR(10) primary key, Sname VARCHAR(10), age SMALLINT, Cname VARCHAR(20) ); 请用ALTER TABLE语句设置age字段默认值为20,正确的语句是()。
A、ALTER TABLE Student ALTER COLUMN age SET DEFAULT 20;
B、ALTER TABLE Student CHANGE COLUMN age SET DEFAULT 20;
C、ALTER TABLE Student ADD COLUMN age SET DEFAULT 20;
D、ALTER TABLE Student DROP COLUMN age SET DEFAULT 20;