Englishfor English speakers
opt
Verb
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If you opt for something, you choose it instead of something else.
With higher gas prices, more buyers are opting for smaller cars.
I wanted to go with them, but opted instead to stay home.
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If you opt out of something, you choose not to do it.
You will continue to receive the email unless you opt out.
in
Preposition
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Used to show that something is inside something else.
The cat is in the box.
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Used to show that someone is at home, or is available.
Is John in?
The Doctor is now in.
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Used to show movement towards the inside.
The rain came in through the window.
clause
Noun
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A clause is a section in a contract or other legal document.
The author's contract contained a clause giving him creative control over the films.
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A clause is a group of words that usually includes a subject and a predicate. In English, the subject usually comes before the predicate.
The last clause in the sentence is hard to understand without knowing some background.