Had the weather ______ better, we would have started the outdoor experiment as planned.
答案解析
核心考点：虚拟语气倒装结构。解题思路：'Had...been'为省略if的倒装结构，正确选项B保持过去分词形式。错误选项A/C/D均不符合虚拟语气中条件句的语法要求。易错点：混淆倒装句的谓语动词形式。
正确答案：B