He said he would go to the dress rehearsal, （ ） he was not too b

He said he would go to the dress rehearsal, （ ） he was not too busy.
A、provided
B、unless
C、whether
D、thus
【正确答案】：A
【题目解析】：provided: 如果。unless: 除非，如果不，含有否定意味。whether：是否。thus：因此。根据语境判断，前后句是因果关系，如果他不忙，就会去参加彩排。所以A选项符合语境。句意：他说只要他不太忙，他就去彩排。