The professor said he could talk on _____ interested the audience.
D
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At______,photographyisachemicalprocess,duringwhichalight-sensitivematerialisalteredwhenexposedtolight.
John'sscoreonthetestisthehighestintheclass;he____lastnight.
—Haveyoufinishedthework?
—Notyet,butnomatterhowharditis,we'llkeep___untilwemakeit.
Ifasubstanceisdissolvedinwaterorheated，itmay（）agas.