What is the correct depth to press for compressions

Driving Theory test question
An adult casualty is not breathing. To maintain circulation, compressions should be given. What is the correct depth to press?
A) 1 to 2 centimetres
C) 10 to 15 centimetres
B) 4 to 5 centimetres
D) 15 to 20 centimetres

Answer: B
Explanation:
An adult casualty is not breathing normally. To maintain circulation place two hands on the centre of the chest. Then press down 4 to 5 centimetres at a rate of 100 times per minute.