There are hair cells in the inner ear that are involved in your ability to hear. Excessively loud noise can kill these cells, and the longer you’re exposed to the loud noise, the greater the number of cells that are destroyed. The greater the number of cells that are destroyed, the worse your hearing gets. Once those hair cells have died, they stay dead; so your hearing isn’t likely to get better in the future. What is the topic sentence of this paragraph?

Once those hair cells have died, they stay dead; so your hearing isn’t likely to get better in the future.

There are hair cells in the inner ear that are involved in your ability to hear.

Excessively loud noise can kill these cells, and the longer you’re exposed to the loud noise, the greater the number of cells that are destroyed.

The greater the number of cells that are destroyed, the worse your hearing gets.