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Q; What is the longest word all of whose contiguous subsequences are words?

A:

 AMUSERS (7)

 AMUSERS->AMUSER->AMUSE->AMUS->AMU->AM
 MUSERS->MUSER->MUSE->MUS->MU
 USERS->USER->USE->US
 SERS->SER->SE
 ERS->ER
 RS

All words are in Webster's Third unless otherwise indicated.
Here are definitions for the less common words:

 AMU: a unit of mass, formerly an acronym for Atomic Mass Unit [[[RH2]]
 MU: a Greek letter
 SER: a unit of weight in India, a variant of SEER [[[RH2, SOED]]
 SE: a Chinese zither [[[SOED]]
 ERS: ervil [[[RH2]]
 ER: interjection used to express hesitation [[[RH2, SOED]]

 RH2 = Random House Unabridged Dictionary, Second Edition
 SOED = Shorter Oxford English Dictionary
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