The correct answer is A. Deuterium oxide.
Deuterium oxide, also known as heavy water, is a form of water that contains a heavier isotope of hydrogen, called deuterium. Deuterium oxide is a colorless, odorless, and tasteless liquid that is slightly denser than regular water. It is used in a variety of industrial and research applications, including nuclear power generation, medical imaging, and environmental monitoring.
Option B, pH7, is the pH of pure water. pH is a measure of the acidity or alkalinity of a solution. A pH of 7 is neutral, a pH below 7 is acidic, and a pH above 7 is alkaline.
Option C, rainwater, is water that has fallen from the sky as rain. Rainwater is typically a mixture of water vapor, atmospheric gases, and particles from the Earth’s surface.
Option D, tritium oxide, is a radioactive form of water that contains the isotope tritium. Tritium oxide is used in a variety of research applications, including nuclear power generation and medical imaging.