【正确答案】 C

【答案解析】[解析] 划线词词义是“许多，大量”。
A项gradually意为逐渐地；例句：The rain lessened gradually and gave over at last.雨渐渐小下来，终于停止了。
B项suddenly意为突然地；例句：Suddenly he began to shriek loudly.突然他开始大声尖叫起来。
C项greatly意为大大地。例句：His writing has improved greatly in this semester.这学期他的写作有了很大进步。
D项frequently意为频繁地；例句：He comes to visit me frequently.他经常来看我。
很明显C项正确。