George had floated through his classes at Stovington. A soccer and baseball star, his academic program had been fairly undemanding and he had been content with C's and an occasional B in history or botany. He was a fierce field contender but a lackadaisical, amused sort of student in the classrooms Jack was familiar with the type, more from his own days as a high school and college student than from his teaching experience, which was at second hand. George Hatfield was a jock. He could be a calm, undemanding figure in the classroom, but when the right set of competitive stimuli was applied (like electrodes to the temples of Frankenstein's monster, Jack thought wryly), he could become a juggernaut.
Q 1: What's the meaning of 'C's?'
Q 2: when the right set of competitive stimuli was applied == when the right set of competitive stimuli was applied in the classroom or in the field? why?