A.has increased toB.have increased to

C.has increased byD.have increased by

【答案】C

【解析】

考查时态、主谓一致和固定短语。句意：从那时起，上驾驶课程的人数增加了20%。根据“ since then”可知，本句为现在完成时，主语为the number of people，谓语动词为单数形式。increase to “增加到”，increase by“增加了”，再结合句意。故选C项。