It is certainly? or it certainly is?

Hello,

I found this dialogue in an old book "Telephoning in English":

A: Thanks for your letter of the 24th May. I got it today and I'm phoning to save time.

B: Ah yes, thanks. So is it alright if I come to your place next week?

A: It certainly is.

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I wonder why we don't say, "It is certainly."

Please help!

Thank you!