Year 12s: what will be the first way you research your future options? >>

Trouble understanding these sets! / Semantics

Hi guys! I'm not sure with some answers for the following 5 sets, it would be amazing if someone could help! I'm mainly getting confused with the bracketing.So the Sets are: S1= {{B}, B} S2={{B}}S3={B}S4=B={e,f,g}And the questions I have... 1. What is the cardinality of S1? I thought it would be 4, as the second B is equivalent to S4 which means it has 4 members. My friend however thinks its 2!2. What is the Union of S1 and S2? I thought it might be S1US2= {{B}, B, {{B}}}.3.What is the complemet of S1 in S2? S2\S1 = {{B}} or nothing? 4. What are the subsets of S1? Is it both {A} and A or only {A}? I think it is {A} because this only contains 1 member but I'm unsure!5. Which are the subsets of S2? Do I write {{A}} or {A}? I know a set can be a subset of itself but do I leave the outer brackets or keep them on? Thanks soo so much in advance! Really feel like I'm struggling a bit and we have a test tomorrow so any help is much appreciated!