GMAT Sentence Correction (SC) | Re: Scientists have recently discovered what could be the

Following are the cases whencomma + and is used:

1. Before the last item in a list of three or more items:
I like apples, oranges, and grapes.... correct
However this usage is optional. Even without the comma, the sentence is correct: I like apples, oranges and grapes.... correct

2. To join two independent clauses:
She sings, and she dances.... correct.
Here the usage is mandatory. Without the comma,
...

---