Hey Ana, how are you? I hope you are doing just fine! Firstly, I should like to congratulate you upon posting such an interesting question on the italki board here. In second, you should know that the word ''since'' has got a broad usage and even different meanings depending on the context. However, by considering the context concerned, I shall provide you with a little information on the usage of this word: 1) As you may have already learned, the word ''since'' is used to say that something has been true from a particular time in the past until now. E.g.: ''Exam results have improved since 2001.'' E.g.: ''I've been wearing glasses since I was three.'' * In sentences like these you use 'a perfect form' with 'since'. Don't say ''Exam results improved since 2001'' or ''I am wearing glasses since I was three''. --> You can also use ''since'' to say how long ago something happened. When you use ''since'' like this, you use 'a simple form.' For example, instead of saying ''I last saw him five years ago'', you can say ''It's five years since I last saw him''. E.g.: ''It's three months since Kathy left.'' (or ''It has been three months...'') E.g.: ''It's years since I heard that song.'' (or ''It has been years...'') So, back to your sentence in the beginning (It’s a long time since we ................. to the theatre) and your question regarding the choice between the options ''A'' or ''B'', I should tell you that ''B'' (went) is the correct answer to be marked and with which to fill in the blank. I hope I have helped you! Should you have any further questions or doubts regarding English Grammar, do not hesitate to contact me. I shall be pleased to help!

Have been, is correct it is the present perfect tense used when we want to say something that started in the past is still continuing today. 'A long time since,' tells us it started in the past and is still true today. Went is the past tense of go and doesn't fit. We haven't gone to the theatre for a long time. We went to the theatre yesterday.

The great and awful thing about english, and other languages, is that there are so many ways to say the same thing. As a native english speaker i dont hear any difference between the two options but if i had to pick, i would pick choice B. Choice A is definitely more casual/colloquial, however you can make choice B less formal by combining "we" and "have" to make "we've". For example, "it has been a long time since we've..." this is te way that i would say it. Sorry i couldnt explain any better!

What do you think?