The velocities of two arbitrary points A and B on a rigid body are vA and vB, respectively. Which statement is correct（）
A. If the rigid body is in translation, then ½vA½=½vB½ must be satisfied
B. If ½vA½=½vB½ is satisfied, then the rigid body must be in translation
C. If the rigid body is in translation, then ½vA½=½vB½ must be satisfied and the directions of the two velocities could be different
D. If the rigid body is in translation, then the directions of vA and vB must be the same but probably |vA|¹|vB
The theorem of composition of velocities（）
A. is only valid when the transport motion is a translation
B. is only valid when the transport motion is a rotation
C. is not valid when the transport motion is a rotation
D. is valid for any type of transport motion
Which of the following is definitely true, according to the passage（）
A. The young people are studying too hard
B. The courses in the youth center are too difficult
C. The government financed the establishment of job-oriented school
D. The tests given to the youngsters are too difficult
Of the following four immunization routes, which one is the best route for protein antigens（）
A. oral administration
B. intramuscular injection
C. intravenous injection
D. intradermal injection
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