Question

---It looks as if a storm is coming up. ---Yes,____ A.it certainly does. B.it looks so

答案是A，那B呢？

Answer 1

A（这是答案）
it certainly does=it certainly looks as if a storm is coming.类似于一般疑问句中的简单回答。
Yes, it does.
B.
it looks so:
如此，这样：
一般被认为是代词，代表前文提到的情况。但你这是as if从句，所以，不能so代替as if从句。

追问

为什么so不能代替as if从句

追答

ADV 如此，这样(指已经提到的事) You use so to refer back to something that has just been mentioned.
'Do you think that made much of a difference to the family?' — 'I think so.'...
“你认为那会给这个家庭带来很大的变化吗?”——“我认为是这样的。”
If you can't play straight, then say so...
要是你无法做到诚实坦率，就直说吧。
'Is he the kind of man who can be as flexible as he needs to be?' — ' Well, I hope so.'...
“他是那种能够灵活变通的人吗?”“嗯，希望如此。”
so没有“as if 从句”似乎的意思。

参考资料： http://www.iciba.com/so/

Answer 2

回答翻译成：是的，的确如此。这是固定搭配。语法如下：
例如So I am told. 我也这样听说过
“的确如此”翻译成英语就是so it is.
例如, A说He is a smart boy.B可以说so it is来表示赞同，肯定的意思

Answer 3

B不是地道的英语，中式的