Regarding your a,b,c and d versus a,b,c, and d: Look up and learn about the “Oxford comma”.
Without the comma, “c and d” leaves an ambiguity regarding whether “c and d” are considered to be one item, whereas using the Oxford comma leaves no doubt as to the separate status of “c, and d”.
Precision and accuracy are the reasons for using “it”.
I’ve never understood why parentheses are acceptable in mathematical groupings but not written text; most likely there is no intonation/inflection to indicate such.
I know you guys are correct about “, and”. I’m probably dating myself but it’s just isn’t the way I was taught. And it could be that the rule has been in place for a long time and I had bad teachers.
BTW, is it OK to start a sentence with AND? It wasn’t allowed by my teachers but I do it all the time. Same with sentence fragments.
Maybe I should go back to school.
My
Dear
Aunt
Sally