Acting as a reference point for detecting motion
Period are going left to right across the periodic table
Groups are going up to down on the periodic table
Pressure: when the external pressure is:
less than one atmosphere, the boiling point of the liquid is lower than its normal boiling point.
equal to one atmosphere, the boiling point of a liquid is called the normal boiling point.
greater than one atmosphere, the boiling point of the liquid is greater than its normal boiling point.
C would be the right answer