B着 （还可以说 “着本”） A没准儿 （意思是：说不定） B不一定 （不要等他，说明他不一定来 ，否则：没准儿参加得了，说明他会来，干嘛不等他呢？）

2.1.自 习 时, 我 手 里 拿 ------------英 语 书 ， 可 是 心 却 不 在 书 上 。 B.着 means take,although I took it,but my eyes were not on it. emphasizes take. If use A.it emphasizes the book,coz ben is measure word of shu(book) 2.2. 下 这 么 大 雪 ， 他 今 天 -------------不 来 了。 A.没 准儿 A means not sure abt that. B,不一定，不一定不来了，it's not right~ can't not make them together.or you can say,他今天不一定来。without 了。 2.3. 今 天 的 晚 会，他 ------------------参 加 得 了， 我 们 不 要 等 他。 B.不 一 定 B means not must be, both A and B mean maybe,but they are used with different structures.If you use 没准儿here, you could say,他没准不能参加了。

b a b

1.B.If you want to choose A,the sentence must be like this:我手里拿着一本英语书,"本"can not be used without a measure word.But the word"着"used here means that you are holding the English book now. 2.A."没准儿"means you are not sure,now pay attention,"不来了"are negative words."没准儿"can be used with negative words.But"不一定"can not . 3.B."参加得了"are assertive words.So A and B can be used with them.But if you choose A,the sentence means he is very likely to come,we should not wait for him.——Of course ,it's wrong.IF you choose B,the sentence means he is not sure to come or not,so we should not wait for him.So,choose B is reasonable. Hope my answer can help you.