Yes, the system puts in random words for the 'wrong' answers. Sometimes they are valid - which is unfair.
But sometimes they are invalid, but the level of Gaelic required to know they are invalid is quite high. In this case we have covered it, but it is still a bit mean. 'That building' would be An togalach sin. * Togalach sin would be something like '* Building there' (with building as a noun) which does not make any sense..
This is to do with the psychology of language. It doesn't sound like an f you are familiar with so you interpret as something else. Having listened to it carefully (which anyone using the website can do by clicking on the Tog in blue then finding the correct sentence and clicking on the speaker) I can assure you it is a normal pronunciation and I recognised it immediately as an f. However, when I tried to characterize it, it is definitely not an English f. I think possibly it may be made with both lips, and possibly with the voicing of the vowel starting sooner than you are used to. I know this is quite technical, but it means, "Yes, it may be nearer to a b than you are used to." You will soon get used to it. D