*Question*
Q: Mammals are synapsids, meaning they have a single opening in what structure? Choices: - spine - heart - hip - skull A:
**Answer**
skull

*Question*
Q: The ostwald process is the commercial method for producing what? Choices: - citric acid - deoxyribonucleic acid - chlorine - nitric acid A:
**Answer**
nitric acid

*Question*
Q: Earth is the only planet in the solar system that has what element, which is essential for human life, present in all three of its states? Choices: - helium - oxygen - carbon - water A:
**Answer**
water

*Question*
Q: What group of gases are unusually unreactive? Choices: - volcanic gases - greenhouse gases - halogens - noble gases A:
**Answer**
noble gases