1. It's funny, and I can't explain why, but it's true that "wide knowledge" doesn't sound quite right to my ear, although it appears to be a logical thing to say. You also could say "broad knowledge", "deep knowledge" or indeed "extensive knowledge". 2. Yes, you can have a large quantity of things, in particular if that quantity is measured by something other than its number, thus weight of potatoes, volume of mussels, and so on.

1. The better word choice is "extensive" but "wide" isn't *wrong*. The adjectives James provides are all far more natural/common. More likely you will hear "wide-ranging" knowledge. 1a. If the exercise key really says "knowledge on Astrophysics" I would not trust the author's knowledge of English very much (it's "knowledge of astrophysics"). 2. Quantity can be used with both countable & uncountable nouns, it is not always natural but it is grammatical: - A quantity of meat. - A quantity of steaks. - A quantity of water. - A quantity of water droplets. - A quantity of sand. - A quantity of sand castles. - A quantity of knowledge. - A quantity of thoughts.

1. Both 'wide' and 'extensive' are acceptable adjectives for use with the word knowledge. They both convey a similar level of intensity. 2. Some sources differentiate between 'number' and 'amount', with 'number' used for countable nouns and 'amount' for uncountable, but in practise this difference in use is not so rigid.

1. Wide is to show measurements. Example : the door us wide.