请问It's____that I'd like to take a walk.填such nice weather为什么

请问It's____that I'd like to take a walk.填such nice weather为什么错?
A.so nice weather B.such nice weather C.so nice a weather D.such a nice weather
答案选择A,我明白为什么A是正确的.
但是不明白B为什么不正确.
这是课标本教材完全解读（王后雄学案）上的题目.就在旁边还写着It was so warm a day that we decided to go to the sea.=It was such a warm day that we decide to go to the sea.
但是我认为B也是对的.就像such a warm day,只是这里weather不可数,所以不加a.
请高手给予解释,谢谢.
不好意思,说反了.答案选择B,我明白为什么B是正确的.但是我觉得A也正确,这个相当于so warm a day,只是这里weather不可数,所以不加a.