Which sentence uses the word "then" correctly? Use the "Mechanics and Usage Resource" if needed.

I enjoyed my job at the animal shelter better then any other job I've held.
It was easier to learn the skills I needed for this job then it was to learn other job skills.
Due to their size, cleaning dog pens was more difficult then cleaning cat pens.
Once I'd learned how to clean pens, then I was allowed to learn how to feed the animals.