A、They are liked by the same group of people. B、They are always done at the same time. C、They can’t be separated from each other

Many people who drink alcohol also like to smoke cigarettes. Drinking and smoking seem to be closely linked. Some people might even say they go together hand-in-hand.
    But this may be more than just a mixing of two fairly common activities.
    New research has looked more closely at the relationship between cigarettes and alcohol. And the relationship is one of equal dependence. Smoking and drinking seem to feed on each other.
    These are the exact words of Mahesh Thakkar. He is head of research in the Department of Neurology at the University of Missouri’s School of Medicine.
    Thakkar explains that when a person drinks alcohol they get sleepy. He says a drug in cigarettes fights that sleepy feeling. That drug, nicotine, can be addictive; smokers need more cigarettes because of their body’s growing dependence on the drug.
    So, if a person smokes, then he or she is much more likely to drink alcohol, and vice versa. Thakkar says, "They feed off one another."
    Researchers already knew that people who use alcohol often smoke. In fact, earlier research shows that more than 85 percent of alcohol-dependent American adults also depend on nicotine.
    Thakkar’s earlier research showed that nicotine combined with alcohol stimulates what he calls the "reward center" of the brain. However, the new study shows a dependent relationship between the substances.
    Thakkar says his team "found that nicotine weakens" the sleep-causing effects of alcohol. It does this by activating an area of the brain called the basal forebrain. According to the Psychology Definition website, that area is responsible for memory, learning and attention.
    For this new experiment, Thakkar and his team used equipment that measures brain activity in rats. They injected the rats with both nicotine and alcohol, and then studied brain activity in the animals as they slept. The researchers found that nicotine goes through the basal forebrain and cancels out the sleep-causing effects of alcohol.
    So, why is this new research important?
    The World Health Organization says 7 million people die every year from alcohol and nicotine use.
    Mahesh Thakker and his team identified why alcohol use and smoking are often linked. He says this knowledge may help people break their addictions to alcohol and nicotine.
    The researchers published their findings in the Journal of Neurochemistry.
20. What is the relationship between smoking and drinking?
21. What do we know about nicotine?
22. What is the significance of the new study?