3. Solar panels are composed of smaller units called (1) ______. In a solar cell, crystalline silicon is sandwiched between (2) ______ layers.

3. Before the 20th century, most people knew how to assess freshness using (1) ______, smell, and touch. Many foods are safe to eat far beyond their labeled (2) ______. Low freezer temperatures keep bacteria that cause food (3) ______ in check.

3. One geographic concept that could help you in a zombie apocalypse is (1) ______. Push factors will make you want to (2) ______ somewhere. If a zombie (3) ______ started, it would be safer to start in a city that does not have a direct connection to other cities.