Question

All these sentences use the word "so" correctly except: Billy set off across the dusty field. So did his trusty ferret Gonzo. Billy set off across the dusty field, and so did his trusty ferret Gonzo. Billy set off across the dusty field, so did his trusty ferret Gonzo. Billy set off across the dusty field; so did his trusty ferret Gonzo.

Answer 1

Billy set off across the dusty field. So did his trusty ferret Gonzo.
there should have been a coma there instead of a period

Answer 2

The first sentence uses "so" wrong.