Modify existing table

Examine the structure of contact table:
CONTACT
(
CONTID NOT NULL NUMBER(3),
NAME VARCHAR2(25),
ADDRESS VARCHAR2(50),
BIRTHDATE DATE
)
Currently the table is empty. You have decided that null values should not be
allowed for the NAME column. Which statement restricts NULL values from being
entered into column?

A. ALTER TABLE CONTACT ADD CONSTRAINT NAME(NOT NULL);
B. ALTER TABLE CONTACT ADD CONSTRAINT NOT NULL (NAME);
C. ALTER TABLE CONTACT MODIFY CONSTRAINT NAME(NOT NULL);
D. ALTER TABLE CONTACT MODIFY(NAME VARCHAR2(25) NOT NULL);

Answer:[D]
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