A/an works by increasing activity in certain areas of the brain to increase concentration and wakefulness. a. stimulant b. hypnotic c. antipsychotic drug d. barbiturate

Option A stimulant

Explanation: Stimulants are those class of drugs which have effect on central nervous system. These drugs increase the activity of CNS and the body. Such drugs are given to patients suffering from attention-deficit hyperactivity disorder. It causes the patients to focus more, and elevate the mood of the patients. Appropriate amount of these drugs is effective, higher doses cause decrease in activity.

every el niño cycle is different. every year.

answer:

sequence 1- deletion

                  2- duplication

                  3- insertion

                  4- substitute

                  5- inversion