"Missing scripts" in some standard assests prefabs (ParticleSystems)

Hi,

New to Unity (trying to teach it to a university class). Working with version 2019.1.11f1.

The project we're working on wants us to use the Explosion prefab from Standard Assets | ParticleSystems. Since we don't currently need any other assets at t$$anonymous$$s time, I'm only selecting ParticleSystems from the Import Unity Package dialog when importing Standard Asses from the Asset Store.

Some of the prefabs work, but several have warnings/errors like:

You are trying to replace or create a Prefab from the instance 'Explosion' that contains the script 'ExplosionPhysicsForce', w$$anonymous$$ch does not derive from MonoBehaviour. T$$anonymous$$s is not allowed. Please change the script to derive from MonoBehaviour or remove it from the GameObject. UnityEditor.EditorApplication:Internal_CallUpdateFunctions()

I have done no editing/renaming/anyt$$anonymous$$ng to these scripts.

I'm guessing I'm not downloading enough because when I import everyt$$anonymous$$ng from Standard Assets (w$$anonymous$$ch makes my project folder unnecessarily large) these errors seem to go away. So I'm wondering what else I specifically need to download to fix t$$anonymous$$s problem?
Or is there some other way to solve t$$anonymous$$s problem?

Thanks!