Bob promises to join in the football match he has to help his parents on the farm.A. ifB.

问题补充：

Bob promises to join in the football match he has to help his parents on the farm.

A. if

B. as

C. unless

D. when

答案：

A. if

B. as

(答案→)C. unless

D. when

解析：if 如果，是否；as由于，作为；unless除非，如果不；when既然，当……时。如果他在农场上帮助他的父母，鲍勃许诺帮助参加这场足球比赛。故选C。