Which condition would cause the altimeter to indicate a lower altitude than true altitude?

1) What is pressure altitude

2) Which of the following defines the type of altitude used when maintaining FL 210?

3) Altimeter setting is the value to which the scale of the pressure altimeter is set so the altimeter indicates

4) Which condition would cause the altimeter to indicate a lower altitude than true altitude?

5) Under which condition will pressure altitude be equal to true altitude?