Because of too many differences between the two sides a lot of questions are still left .

问题补充：

Because of too many differences between the two sides, a lot of questions are still left .

A in the airB on the airC by the airD through the air

答案：

解析：本题考查词组搭配。In the air 意思是“（计划等）未决定的”；on the air 意思是“在广播中”；C、D不正确。根据句意“很多问题还未解决”，故A正确。