第21题: [单项选择]People seldom trust him since he usually fails to______his promises. A. keep to B. keep on C. keep off D. keep up 参考答案:A 答案解析:解析:本题需弄清以keep为中心的四个短语动词的意思。A项keep to意为"遵守,坚持",B项keep on意为"继续…",C项keep off意为"让开,挡开",D项keep up意为"保持,继续"。根据题意,A为正确答案。