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1. They picked fruit and ate it.

2. They picked fruits and ate them.

3. Tropical fruit differs from that from cold countries.

4. Tropical fruits differ from those from cold countries.

5. Races become accustomed to local food and refuse substitutes.

6. Races become accustomed to local foods and refuse substitutes.

Which of the sentences are grammatically correct? I find them confusing.
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They are all correct, just in one instance using a singular form and in the other a plural one. Fruit can be used a a generic for any fruit, fruits implies there is more than one variety available.
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They are all theoretically ok, but as Clive points out, we rarely use 'fruits' or 'foods' and we don't use 'race' in that way.
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Comments  
Hi Yoongliat,

Here are a few general comments on usage, not grammar, that I hope may be of interest to you.

We use the word 'fruit' much, much more commonly than the word 'fruits'.

The word 'race' is not normally used in this way, for several reasons. eg

People where I live are reluctant to mention race for fear of seeming to be discriminating in some inappropriate way.

We see races as spread throughout many countries, eg there are Chinese people all over the world.

Rather than 'race', I suggest you'll hear people say something like People become accustomed to their local foods and refuse substitutes.

Best wishes, Clive
Thanks, Clive.

Feebs11 says that all the sentences are correct, but I believe a couple of them are wrong.
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 nona the brit's reply was promoted to an answer.
nona

Many thanks.