Crisis on Infinite Earths: Part One - S5-E9
Trivia: One question Marv Wolfman continues to get asked at fan conventions is why he killed Kara off in the "Crisis" comics. With so many characters established as having survived Krypton's destruction - including Clark, Kara, Krypto the Superdog, criminals banished to the Phantom Zone, and the bottled Kandor - his goal was to make Superman unique as the planet's sole survivor; killing Kara gave him the creative freedom to return Clark to his origin as the Last Son of Krypton.
Answer: For one, it's usually cheaper to film in Vancouver because of the tax breaks. But all the CW Arrowverse shows (with the exception of "Black Lighting") are/were filmed in Vancouver. So it makes it easier and cheaper for sharing set designs, crossovers, etc ("Black Lightning" filmed in Vancouver during its crossover). Originally "Supergirl" wasn't part of the Arrowverse when it was on CBS, but it was too expensive to produce and wasn't reaching the wider audience the CW shows had. So CBS moved it to its sister company, The CW.
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