Question

why do we use `but` and `or`?​

Answer 1

Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's metabolic activities.

Answer 2

We use but as an alternative to except (for), apart from and bar to introduce the only thing or person that the main part of the sentence does not include. It is often used after words such as everyone, nobody, anything, anywhere, all, no, none, any, every.

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Or is a conjunction that connects two or more possibilities or alternatives. It connects words, phrases and clauses which are the same grammatical type: Which do you prefer? Leather or suede?