Relatives clauses

Fill in the blanks, using the appropriate relative pronoun. If you think the relative pronoun can be omitted, write "?".
Smokers believe they can avoid fatal disease by slashing the number of cigarettes they smoke each day are mistaken, according to a new study suggests the only way to escape the risk is to quit.
The findings will add to concerns about the way tobacco companies are encouraging smokers to alter their habits rather than quit.
Nicotine Replacement Therapy (NRT) has become common in hospital intensive care units (ICUs) in the last five to 10 years. The drug reduces withdrawal symptoms, such as headache and irritability, among smokers in these units are too sick to go to an area they can smoke.
The team examined the intensive care records of 227 smokers, half of received
NRT, mostly via skin patches.