ceilidh.erickson wrote:The frustrating thing about comparisons is that often there is more than one comparison structure that would be correct. For example:

Pizza tastes better than salad.
vs.
Pizza tastes better than salad does.

These are both grammatically correct. In the first example, we're comparing nouns, but in the second we're comparing the act of tasting. Because the meaning is the same in either case, the "does" is not necessary in the second example.

There are times when a verb is necessary in a comparison, though. Consider:

I want to eat a hamburger more than my dog.

Well, this could mean that I want to eat a hamburger more than my dog wants to eat a hamburger, or it could mean that I want to eat a hamburger more than I want to eat my dog! I'd need to add something to make it clear whether "dog" is the comparative subject or object:

I want to eat a hamburger more than my dog does. -> My dog also wants to eat a hamburger.
I want to eat a hamburger more than I do my dog. -> I want to eat my dog.

If we need to make clear whether we're comparing a SUBJECT or an OBJECT, then it's necessary to include a verb. If the meaning is unambiguous, we don't need to include a verb.

In your example, "wild animals have less total fat than livestock," there is no ambiguity - we're comparing wild animals to livestock. (We do have an object here, "total fat," but it would be nonsensical to say "wild animals have less total fat than they have livestock," so there is no meaning ambiguity). As such, we don't need a verb. Often when something is not grammatically necessary, the OG explanation will say that it can be omitted. That's not to say that it would be grammatically incorrect to include the "do." It would just be less concise than it could be.

Does that help?

ceilidh.erickson wrote:

Crystal W wrote: Thank you for you explanation, but can you explainmore about choice A? I know the mistake about they in choice A, but do you think A should omit do or put do after livestock, because the do refers to have?
Thanks in advance!

I think it's actually strange that the answer explanation cited the "do" issue specifically. It's perfectly correct with it, perfectly correct without it. You won't ever see a question that simply comes down to this issue, when there's no meaning distinction between the two.

aflaam wrote:

ceilidh.erickson wrote:

Crystal W wrote: Thank you for you explanation, but can you explainmore about choice A? I know the mistake about they in choice A, but do you think A should omit do or put do after livestock, because the do refers to have?
Thanks in advance!

I think it's actually strange that the answer explanation cited the "do" issue specifically. It's perfectly correct with it, perfectly correct without it. You won't ever see a question that simply comes down to this issue, when there's no meaning distinction between the two.

Hello Ceilidh.
In this SC are do and have replaceable?
wild animals have less total fat than have livestock fed on grain
wild animals have less total fat than do livestock fed on grain
Are these two correct?
if yes, then Is this always the case in comparisons?
Thanks