四、閱讀測驗   Since many people now value unique life experiences over material goods, visiting the world’s mostbeautiful locations has become an increasingly common passion. As a result, the tourism industry has beengrowing at a yearly rate of around 4 percent. Though we may be living in a golden age of travel, masstourism—and the frequent air travel that goes along with it—could have a severe effect on the environment if leftunchecked.   The millions of feet that trample on the world’s natural wonders can do as much damage in a year as naturalcauses would in hundreds, not to mention the tons of garbage left by careless tourists. Even more insidious is thelarger amount of carbon emissions produced by modern airplanes. In 2018, 4.3 billion passengers traveled by air,which was 38 million more than the previous year. Its effect was once almost negligible, but now air travelaccounts for 2 percent of global carbon emissions.   For those of us concerned about the environment, what can be done to make sure our travels are asenvironmentally friendly as possible? Travelers to environmentally vulnerable places must of course take care tolimit the direct impact of their visit—that is, do their best to respect local laws and leave no trace. However, evenmore vital is to carefully research any trip and only choose destinations that have the infrastructure necessary tohandle tourism in a sustainable way. If traveling by air is necessary, passengers should pack lightly, which saveson airplane fuel, and whenever possible, fly directly rather than with several layovers. Airplanes use a largeamount of their fuel during takeoff, so avoiding multiple stops will considerably reduce a passenger’s personalcarbon footprint.Secondly, if you can afford it, take advantage of “carbon offsetting” choices offered by many airlines. Thisis usually an extra charge that goes toward funding schemes that reduce your carbon footprint, such asforest-planting or clean-energy projects.Traveling the world is something few of us are willing to give up. The least we can do is make sure that ourown enjoyment does not make it impossible for the next generation of travelers to enjoy the world.
21. What does “if left unchecked” mean in the first paragraph?
(A) if not locked
(B) if none attended
(C) if left by itself
(D) if not to be restricted