We are going to have a barbecue ------- it rains

选D unless 除非
A if not意思是如果不,但是不能直接用 if not it rains,而应该是 if it does not rain
B when 当, 当下雨我们去烧烤,显然不对
C except that 是除了……,此处不适用.例句：Your homework is good except for some spelling mistakes. 你的作业不错就是有点拼写错误.