Air is a mixture because it is odorless at low temperatures. Why is this statement
wrong?

Combustion is the act or process of burning. therefore, i believe the answer is  a: burning wood. 

Each calcium ion has a charge of +2, which can also be written as ca2+. if those ions move out of the neuron and no other ions are moving, the neuron would lose the positive charge associated with those ions and become more negatively charged.

The answer is the third option extremely gradual