why do we need the comma before "and" ? maybe someone can help...
... With correct choice (D):
Geologists believe that the warning signs for a major earthquake may include sudden fluctuations in local seismic activity, tilting and other deformations of the Earth's crust, changes in the measured strain across a fault zone, and variations in the electrical properties of underground rocks.
Good question about commas: in this case it's called a serial comma, and question is whether to use it.
Suppose you have a list that itemizes things like this:
Items on the menu included w, x, y, and z
or do you say,
Items on the menu included w, x, y and z
(without the final "serial" comma)?
The test question above is constructed the same way, it just has phrases in place of w, x, y, and z.
ETS wouldn't test this specifically, because both styles are "correct". It's kind of optional.
Traditional, standard written English recommends that you DO use that final serial comma, to avoid ambiguity. Not a life-or-death issue, but that is the syntactically consistent way to write the sentence. "And" is a conjuntion. The comma is a punctuation mark. They are NOT interchangeable. Just using the word "and" does not in fact eliminate the need for a comma.
But more important, sometimes it may not be clear, if you don't use the comma, whether the "y and z" elements are together or separate ... e.g., is it Fish and Chips, or separate items (1) fish and (2) chips? Using that final comma removes all uncertainty. In a more complex, longer sentence with a lot of and's
in it, things could really get confusing if the punctuation is not tight.
journalists (in the US anyway) have adopted the style of NOT using that final comma in order to conserve one character in their articles, and so people are accustomed to not seeing it. You'd see this, for example, in the "AP Style Guide". In fact, if you asked a reporter or newspaper editor, they would insist that it's "wrong" to use the final comma, because they were trained not to.
They have no idea they have been trained to violate standard usage in that way. In fact I have heard a lot of people insist that it's "against the rules" to use the final comma -- but they are incorrect, and I'm not sure why they think they were ever taught that.
Bottom line is, I use the comma and it always annoys me when it's missing. But it is considered optional.