21．Which tour do you need to book in advance?

　　A．Cherry Blossom Bike Tour in Washington，D．C．

　　B．Washington Capital Monuments Bicycle Tour．

　　C．Capital City Bike Tour in Washington，D．C．

　　D．Washington Capital Sites at Night Bicycle Tour．

　　22．What will you do on the Capital City Bike Tour?

　　A．Meet famous people． B．Go to a national park．

　　C．Visit well-known museums． D．Enjoy interesting stories．

　　23．Which of the following does the bicycle tour at night provide?

　　A．City maps． B． Cameras． C．Meals D． Safety lights

　　　　　　　　　　　　　　　　　　　B

　　Plastic-Eating Worms

　　Humans produce more than 300 million tons of plastic every year．Almost half of that winds up in landfills(垃圾填埋场)， and up to 12 million tons pollute the oceans． So far there is no effective way to get rid of it， but a new study suggests an answer may lie in the stomachs of some hungry worms．

　　Researchers in Spain and England recently found that the worms of the greater wax moth can break down polyethylene，which accounts for 40% of plastics．The team left 100 wax worms on a commercial polyethylene shopping bag for 12 hours，and the worms consumed and broke down about 92 milligrams，or almost 3% of it．To confirm that the worms' chewing alone was not responsible for the polyethylene breakdown，the researchers made some worms into paste(糊状物) and applied it to plastic films．14 hours later the films had lost 13% of their mass - apparently broken down by enzymes (酶) from the worms' stomachs．Their findings were published in Current Biology in 2017．

　　Federica Bertocchini，co-author of the study，says the worms' ability to break down their everyday food - beeswax - also allows them to break down plastic．＂Wax is a complex mixture，but the basic bond in polyethylene，the carbon-carbon bond，is there as well，＂she explains，＂The wax worm evolved a method or system to break this bond．＂

Jennifer DeBruyn，a microbiologist at the University of Tennessee，who was not involved in the study， says it is not surprising that such worms can break down polyethylene．But compared