Choose the best answer to complete the passage.

Some volcanoes are always (16) ______. They are called active volcanoes. Mount Etna in Italy is an active (17) ______. Some volcanoes have not erupted since prehistoric times. These are (18) _______ extinct volcanoes. Most of the Hawaiian Islands are extinct volcanoes. These volcanoes no longer have a hot spot under them. They cannot erupt anymore. Some volcanoes have not erupted for a long time, but they could erupt again. These are called dormant volcanoes. (dormant: temporarily inactive)

Scientists try to figure out when volcanoes will erupt. Studying volcanoes is hard and dangerous work. Scientists drill into volcanoes. They make maps of the inside of the volcanoes. They use satellites to study volcanoes (19) _______ space. Scientists have been able to predict a few eruptions. But it is not (20) _______ to tell what a volcano might do.