Passage One Questions 46 to 50 are based on the following passage. If you think a high-factor sunscreen ( 防晒霜) keeps you safe from harmful rays,you may be wrong.Research in this week's Nature shows that while factor 50 reduces the nurmber of melanomas( 黑瘤) and delays their occurrence,it can't pr them.Melanomas are the most aggressive skin cancers.You have a higher risk if you have red or blond hair,fair skin,blue or green eyes,or sunburn easily,or if a close relative has had one.Melanomas are more common if you have periodic intene exposure to the sun.Other skin cancers are increasingly likely with long-term exposure. There is continuing debate as to how effective sunscreen is in reducing melanomas-the evidence is weaker than it is for pring other types of skin cancer.A 2011 Australian study of 1,621 people found that people randomly selected to apply sunscreen daily had half the rate of melanomas of people who used cream as needed.A second study,comparing 1,167 people with melanomas to 1,101 who didn't have the cancer, found that using sunscreen routinely,alongside other protection such as hats,long sleeves or staying in the shade,did give some protection.This study said other forms of sun protection-not sunscreen--seemed most beneficial. The study relied on people remembering what they had done over each decade of their lives,so it's not entirely reliable.But it seems reasonable to think sunscreen gives people a false sense of security in the sun. Many people also don't use sunscreen properly-applying insufficient amounts,failing to reapply after a couple of hours and staying in the sun too long.It is sunburn that is most worrying-recent research shows five episodes of sunburn in the age years increases the risk of all skin cancers. The good news is that a combination of sunscreen and covering up can reduce melanoma rates,as shown by Australian figures from their slip-slop-slap campaign.So if there is a heat wave this summer,it would be best for us,too,to slip on a shirt,slop on( 抹上) sunscreen and slap on a hat. 46.What is people's common expectation of a high-factor sunscreen?