[Serious] Is a limited nuclear exchange possible?

[Angrboda]

Let's examine four scenarios:

a) Russia uses a nuke to level an important Ukrainian city?

b) Russia uses a nuke on an eastern NATO country, let's assume Poland? (can be tactical nukes)

c) Russia or China uses a nuke to level a lower population U.S. city as a warning?

d) Russia begins using tactical nukes indiscriminately in Ukraine?

Are all of these likely to escalate into total nuclear annihilation, or is it more likely one side or the other will halt the escalation.