Discussion The position vector of a moving particle at a moment is r(x, y The magnitude of the velocity of the particle at the moment is: dr (A) dt √(B) dt (C) (D) dt v dtDiscussion The position vector of a moving particle at a moment is . The magnitude of the velocity of the particle at the moment is: r(x, y)  t r d d t r d | d |  （A） （B） t r d d  2 2 ) d d ) ( d d ( t y t x （C） （D） + √ √