某商品原价a元，现受季节影响，降价b元之后再优惠20%，那么该商品现在的售价是______元（用a，b的代数式表示）．
80%（a-b）或0.8（a-b）．
Yesterday I was walking downstairs when I missed my foot and had a bad fall, nothing serious _______.
A even B either C then D though
平衡梁在起重作业中起什么作用？
