I can't think of an instance where [the exact order of] 'when was' is ever used without leading to a question mark.
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MaXmOuSeAs Cairn says, you need a subject for the verb in the subordinate clause. If you choose 'when was', the only possib
1. Photography changed dramatically ________ introduced instant pictures.
A. when Polaroid
B. Polaroid
C. when was
D. it when Polaroid
Why we have to use "when Polaroid" not "when was" ?