Which of these fits gap 19?

Directions: Complete this passage by selecting the most suitable option for the corresponding gap.
Fire ____(17)_____ things hot by adding heat, and that raises temperature.
You _______(18)______ a fire with its hissing tongues of orange flame.
When something gets too hot and its temperature rises more than it can take, it bursts into hot flames and _____(19)_____ to burn.

Fire is _____(20)_____ than sunlight and it quickly heats up anything it touches.