What's the structure of this sentence?

[Tatyana]

Test No. incompl/advan-34 "Teaching your Wife to Drive", question 10

The only ......... to me is as they were successful on the first attempt, how come some years before it had taken me four tries!

(a) confusion
(b) mystery
(c) intrigue
(d) unknown

Test No. incompl/advan-34 "Teaching your Wife to Drive", answer 10

The only mystery to me is as they were successful on the first attempt, how come some years before it had taken me four tries!

Correct answer: (b) mystery

Your answer was: correct
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Could you please comment on the structure of the sentence itself? Where is the subject and predicate. Thank you.

[Alan]

The only mystery to me SUBJECT is as they were successful on the first attempt, how come some years before PREDICATE it had taken me four tries!
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