Training for Long-Distance Running Causes a Significant Increase in _____

Training for long-distance running causes a significant increase in _____.

A) the cross-sectional area of slow-twitch muscle fibers (cells)
B) the number of myofibrils per muscle fiber (cell) ,increasing cell size
C) the number of slow-twitch muscle fibers (cells)
D) both the cross-sectional area of slow-twitch muscle fibers (cells) and the number of myofibrils per muscle fiber (cell) ,increasing cell size
E) the cross-sectional area of slow-twitch muscle fibers (cells) ;the number of myofibrils per muscle fiber (cell) ,increasing cell size;and the number of slow-twitch muscle fibers (cells)