Which of these fits gap 20?

Directions: Complete this passage by selecting the most suitable option for the corresponding gap.
Lightning and thunder _____ (17) _____ among the first things about nature that mystified and frightened primitive man.
To understand what lightning and thunder actually are, we _____ (18) ______ recall a fact we know about electricity. We know that things become electrically charged – either positively or negatively.
A positive charge has a great attraction for a negative one. As the charges become greater, this attraction becomes ______ (19) _______.
Whatever resistance holds them apart, such as air, glass or other insulating substance, is _____ (20) _____ or “broken down.”