Final answer:
The cause of blisters on flat roofs is air and water trapped between layers.
Step-by-step explanation:
The cause of blisters on flat roofs can be attributed to c. Air & water trapped between layers. When air and water become trapped between the layers of a flat roof, they can cause blisters to form. As the air and water heat and expand, they create pressure that results in the formation of blisters.