Are the "heat engines" fact, theory, or hypothesis? explain

Explanation:

In our age of fuel cells and electric cars, steam locomotives (and even gasoline-powered cars) might seem like horribly old technology. But take a broader view of history and you'll see that even the oldest steam engine is a very modern invention indeed. Humans have been using tools to multiply their muscle power for something like 2.5 million years, but only in the last 300 years or so have we perfected the art of making "muscles"— engine-powered machines—that work all by themselves. Put it another way: humans have been without engines for over 99.9 percent of our existence on Earth!

it's c. elements are placed in rows based on how may valencies electrons have.

with this one i know the answer in craters one reason i looked it up and i learned about it there is my evidence