Identify the first run broadcast network (ABC, NBC, or CBS) of the following TV shows:
Castle, Chicago Fire, CSI, ER, Grey's Anatomy, Scandal, Seinfeld, Magnum PI

In the US, Castle, Grey's Anatomy, and Scandal were first broadcast on ABC
Chicago Fire, ER and Seinfeld were first broadcast on NBC. CSI and Magnum PI were first broadcast on CBS, though Magnum PI switched to NBC for the 5th season of the rebooted series.