What is the mass % of oxygen?

This classification of drug typically changes the brain's chemistry and reduces its ability to create its own endorphins.

Which metal exist in liquid state and can be cut with knife ?

Which process does not require the presence of a physical substance in order to transfer heat? air in the atmosphere is heated by the ground. this warm air then rises, and cooler air falls. this is an example of what type of process? how is conduction different from radiation?