I'm wrong :)) The correct answer is than
And this is the explaination:
Because sooner in no sooner is a comparative adverb like better in no better, the expression should be followed by than, not then: No sooner had she come than the maid knocked. I had no sooner left than she called.
§ 216. no sooner than / no sooner when. 3. Word Choice. The American Heritage Book of English Usage. 1996
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