In article
<fe9732f8-434d-418c-9a4e-1f1aa1d29525@e10g2000prf.googlegroups.com>,
> He is such a good detective that he can catch people's reactions from
> the ___ facial expressions and obtain these very useful clues.
[quoted text clipped - 3 lines]
> The answer my colleague provided was b) fleeting, but my student
> thought a) tremulous may as well make sense. What do you think?
I can't imagine what a tremulous facial expression would look like. Try
some synonyms: quivering, trembling, shaky expressions -- none of these
sounds quite apt to me, but I suppose they could be defended as examples
of synecdoche. Idiom aside, though, I think "fleeting" makes much more
sense, especially in association with "catch" -- timidity is something
more likely to be sensed, perceived, or read.

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Odysseus