씨 is a form of respect that doesn't really exist in English. 님 is considered Mr, Mrs, Miss, Ms, etc.
I wrote. "Mr. Garam, which one do you want?" and it says I used the wrong word :(
Adding the 을 to it allows the ㅅ sound to form a liaison. 인터넷 sounds like inteonet, but 인터넷이 sounds like inteoneshi
For me, this is easier to understand in hangul. 것을 sounds like 거슬. This rule is generally true for batchim sounds when the first consonant of the next syllable is an ㅇ. This is why 맛있어요 sounds like 마시써요 not 맛 있 어 요 :)
씨 is not the same as 님 guys, 씨 is just something you add to the name of someone you're not really close to but its not as "polite" as 님
씨 does not indicate a gender, so "miss" should be a valid answer as well.