Answer:
Withdrawal
Step-by-step explanation:
Chan is experiencing withdrawal. Since he drinks 4 cups of coffee each day, the terrible headache he has that stopped him from concentrating is the Physical and mental symptoms that occur when someone stops or limits his intake of a substance that has become a habit, in this case coffee.
Because taking coffee is already a habit, he had this headache without it because his brain has become used to it and now missing it causes a rush of adrenaline by the brain.