I am a letter. I am in talk, and in stop. What am I ?

I am a letter.

I am in talk, and in stop,
I am in tickle, and in tackle,
I am in bullet, and kite,
I am in poster, and in tear,
I am in torn, and in fear,
I am in tick, and in stock,
I am in tulip, but I'm also in by.

What am I?

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I am a letter, I didn't ask which one.