19. When two beams of light are coherent, which statement is true? (A) Their amplitudes are identical. (B) Their phase difference is fixed. (C) Their propagation speeds are the same. (D) Their propagation directions are the same. (E) All of the above.

1. wo balls enter two frictionless tracks at the same time with the same velocity from the left end. Under the influence of gravity, which ball will arrive at the right end first (A) Ball A. (B) Ball B. (C) Two balls will have the same speed. (D) Uncertain; the answer depends on the height of the slides. (E) Uncertain; the answer depends on the curvature of the right slide.

19. When two beams of light are coherent, which statement is true?
(A) Their amplitudes are identical.
(B) Their phase difference is fixed.
(C) Their propagation speeds are the same.
(D) Their propagation directions are the same.
(E) All of the above.

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