【单选题】What is the difference between a unique constraint and a primary key constraint?
A.
A unique key constraint requires a unique index to enforce the constraint,whereas a primary key constraint can enforce uniqueness using a unique key of or nonunique index
B.
A primary key column can be NULL,but a unique key column cannot be NULL
C.
A primary key constraint can use an existing index ,but a unique constraint always creates an index
D.
A unique constraint column can be NULL,but primary key columns can not be NULL
【单选题】What is the primary purpose of medical education?
A.
For students to learn what it means to be a physician.
B.
To better prepare students for the challenges they will face throughout their professional careers.
C.
Possess a body of knowledge and a set of skills that can be applied in the practice of medicine.
D.
To meet the public's expectations of a doctor.
【单选题】While the Pilot is maneuvering the vessel to a dock,what is the PRIMARY responsibility of the watch officer?
A.
Insure that helm and throttle orders given by the Pilot are correctly executed
B.
Judge the appropriateness of the Pilot's orders and countermand them if necessary
C.
Supervise the signaling and flag etiquette
D.
Record the bells and their times in the bell book
【单选题】要制造一批锉刀,请选择合适的材料( )。
【单选题】For what primary purpose was the Aswan Dam built?
【单选题】What is the main idea of this passage?
A.
The difference between the primary school and the secondary school.
B.
The method that pupils get help from the teachers.
C.
The personal development of the pupils in the secondary school.
D.
The function of the secondary school.
【单选题】What will be the primary responsibilities of the volunteers?
A.
To paint the trash cans.
B.
To pick up and sort the trash.
C.
To pick up and deliver the trash.
D.
Do work in the recycling center.
【单选题】What is the primary difference between the two business models Joost is ignoring?
A.
The number of firms adopting each business model.
B.
Which is the more respectable method to advertising.
C.
Whether to make users pay for what they download.
D.
whether to allow users to keep what they download.