In the U.S., we ask it and answer it like this: Q: What grade are you in? A: I'm in third grade. I know in China, students are always taught: "I'm in grade three." But this is not how we say it. When saying what grade someone is in, we always use ordinal numbers (first, second, third, etc...).

B is correct for both questions. The second question is best answered with "I'm in 3rd grade."

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