13．
Two recent studies might end the great grocery bag debate．The debates over which type of grocery bag is best break down into two issues：which are better for the environment，and which are better for your health．
The first study focused on the question of cloth bag contamination（傳染）．It's well-known that supermarkets offer many opportunities for people to pick up contaminated food：other shoppers contaminate shopping carts，and stockroom clerks contaminate containers．When shoppers put such things into cloth bags，they can contaminate the bag，and then wind up contaminating uncontaminated foods on their next shopping trip．Large numbers of bacteria were found in almost all bags．
Washing the bags might help，but researchers found that"reusable bags are seldom washed and often used for different purposes"．
The second study should end the"use reusable grocery bags"argument．In the study，researchers evaluated（評(píng)估）the bags on a set of environmental standards，including global warming potential．The table below shows how many times that non-HDPE bags must be used in order to bring their global warming potential down to that of an HDPE bag．The first column，for example，shows that a paper bag should be reused three times to reduce its global warming potential to that of an HDPE bag，while you have to use an LDPE bag four times，a non-woven PP bag 11times，and a cotton bag up to 131times to achieve the same end．
| Type of carrier | HDPE bag （No secondary reuse） | HDPE bag （40.3% reused as rubbish bags） | HDPE bag （100% reused as rubbish bags） | HDPE bag （Used three times） |
| Paper bag | 3 | 4 | 7 | 9 |
| LDPE bag | 4 | =5 | 9 | 12 |
| Non-woven PP bag | 11 | 14 | 26 | 33 |
| Cotton bag | 131 | 173 | 327 | 393 |
So the conclusion seems clear：the traditional，thin plastic bag has better environmental performance and is likely to be quite safer for human health．
72．The researchers in both studies might agree thatC．
A．we should reuse our grocery bags as much as possible
B．cotton bags are environment-friendly when they are reused
C．plastic bags are good for the environment and our health
D．grocery bags，if washed，can reduce global warming potential
73．The underlined phrase"wind up"in Paragraph 2probably meansB．
A．give up B．end up C．pick up D．take up
74．What's the problem with cloth bags according to the first study？A
A．They are easily contaminated when reused．
B．They produce large numbers of bacteria．
C．They are poisonous and can pollute foods．
D．They need washing before being reused．
75．We know from the chart that in terms of reducing the global warming potential，A．
A．if an HDPE bag is used three times，a cotton bag should be used 393times
B．the more times HDPE bags are reused，the greater their global warming potential is
C．LDPE bags，when reused 9times，have a better effect than cotton bags reused 131times
D．non-woven PP bags，though reused 33times，work worse than paper bags reused 7times．
