silly, foolish, stupid or ignorant

[Sophie]

Surely no one would be so ... as to pay such a high price for such a poor machine.
a. silly
b. foolish
c. stupid
d. ignorant.
Which is the correct answer?
Thanks.
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On earth there is nothing great but man, in man there is nothing great but mind.

[Nessie]

By the way, please tell me something about the use of "so/as + Adjective + as +to V" (I have only seen this "so/as +Adjective + as + Noun"

Many thanks
Nessie
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... something we never have again, I know... I guess I really really know..

Sorry seems to be the hardest word...

[Yankee]

Hi Sophie

Your sentence is not a good one. Basically, any of the options might be used.

..............

Hi Nessie

I'd say the usual pattern is so+adjective+as+to+verb.

For example:
I found it hard to believe that anyone could be so gullible as to believe that replying to that spam email would make them rich.

That means basically the same as this:
I found it hard to believe that anyone could be gullible enough to believe that replying to that spam email would make them rich.
.